I thought I'd get down a couple of points about common combat mistakes I see in GURPS, and it grew into this.
These are errors I see in character generation, character development, and play. I'm not immune to these, myself, but generally I see them more than suffer from them because I'm the GM.
Betting on Airtight Defenses - I also call this the "perfect defense fallacy."
You see this in character design - maximum defenses, maximum possible HP, maximum DR. Buy the maximum protective perks, advantages, and skills you can before you do anything else. Whenever you have a choice between a game-changer advantage, an offensive advantage, or more defense - choose more defense.
This is a fallacy because you don't always get to defend - some attacks are by surprise, or ignore your chosen defense (and you don't know it does until too late), come from behind, or just affect everyone in an area. Further, some attacks ignore your DR, or just don't let your HP matter much (very high damage attacks, non-damaging afflictions, etc.) Grappling makes some of your defenses harder or negates them, once your foe is in close, and takes away more defenses once a grip is secure.
In GURPS, even a ST 25 barbarian with Mountain of Meat and dwarven plate armor with Fortify +5 on it can be one-shotted and killed outright. Avoiding utility and offensive power and game-changing abilities until your defenses are perfect means you're likely to need perfect defenses. And it won't be enough.
Betting on favorable circumstances.
This is an odd cousin to the perfect defenses fallacy. This is, "my defense bonuses will carry me through if I get all of them. Which I will, almost always."
It's an odd cousin because the perfect defense fallacy attempts to cover all of these bases - get a 16+ on defense rolls even when everything is going wrong. This fallacy assumes you won't have to make do. They overlap where the "won't have to make do" approach means you are buying up all sorts of conditional bonuses (shields, Shield spells, limited DR, specific defenses, etc.) in order to have perfect defenses . . . and then suffer because you need those defenses even when circumstances are not perfect.
Retreat - This warrants a special mention. Depending on Retreat to survive is a common error. Betting on your ability to retreat is a featureless plain fallacy. A substantial minority of the time, you will be unable to Retreat at all. A majority of the time when you're fighting alongside friends, you are better off as a group if no one Retreats. But plenty of players will depend on Retreat, especially in the face of non-striking attacks (slams, grapples, grabs, etc.)
Depending on retrograde movement to make your combat strategy work means "room to move" isn't a luxury, it's a necessity. The Retreat-based strategy also means you depend on your friends to give you space, not support - it's an every-man-for-himself strategy. If your friends are close enough to support you, they are close enough to impede your retreat. Or close enough that if you retreat and they don't, enemies will pour into the gaps.
Defensive fighting is smart, but these three mistakes result from putting defense now (or defense numbers) ahead of all other concerns. It's overdoing a good thing.
This approach can also be frustrating for the player, too - you bank on your top-notch defenses and then along come attacks that make them irrelevant. There is little grumpier than the player of a grappled PC with 1 point in Wrestling, or who is attacked from behind despite 8 front-facing DB and a base Parry of 12, or someone who can't fight without his shield once his shield gets broken. You go from "I can Parry anything 98.1% of the time, and I have luck for the 1.9%" to "this game sucks!" or "We've lost this one" in one move.
The Wunderwaffen Fallacy - Basically, "and then a miracle happens, and we win the fight!"
Summoned monsters, one-shot monster summoning items, potions, alchemist's fire, poison, spells, etc. all fall into this category. Instead of being used as an assistance to the main effort of the group's offense or defense, they often make up the main effort.
Either they get deployed as a first-strike wunderwaffen ("I'll throw this poison gas bomb and sit back and reap the rewards!") or as a last-ditch attempt to save the day ("They're tearing us apart! This statuette that summons a 40-point imp will save us!"), or both. In both cases, they're often deployed with unreasonable expectations of success and thus without support - you send the summoned monster in alone, you throw the grenade in, you change nothing about your approach but down a potion to do it a little better.
In my own games, I have given items that will save the day, and which have done so. But they're vastly outnumbered by the ones that just are helpful to a good strategy but which won't bail you out of a bad one. It's rare to have a one-shot win button magic item or spell or piece of gear, but very often people treat any one-shot item or summoned creature as one.
Not enough offense.
It's one thing to get a solid offensive ability and then diversify. It's another to have, basically, a weak offense.
That generally springs from a few issues.
Not enough offensive variety. Because putting all your eggs in one basket is point-efficient, it's very common for GURPS characters to be one-trick ponies in combat. They end up really bad at everything except their chosen skill, and thus prefer to use that in all cases. It's also monetarily efficient to has one great weapon instead of two not-as-great ones. So resource management pushes for one skill, one weapon.
This is why you see people swinging flails, two-handed swords, shields, and axes while grappled instead of using knives, grappling back, head butting, etc. Even with the penalties for doing so, the other options are often so neglected they won't work in the circumstances they're made for.
A lack of damage variety is another - only cut, only crushing, only impaling, only piercing, etc. - often means you are exceptionally good against opponents vulnerable to that kind of attack. If you are an expert against eye shots, you better hope your opponents have eyes and didn't take Injury Tolerance (No Brain) or its equivalent. If you only have cutting attacks and your foe is vulnerable to crushing, you're giving up an advantage by sticking to one damage type. And so on.
No distance variety. - A subset of this is a lack of ranged attacks. You get the occasional thrown weapon, which are bad against foes with shields or a solid parry skill. But generally guys are "melee" or "ranged" but rarely both. After all, 4 points "wasted" in Bow is a +1 to your melee skill you didn't take, or vice-versa. Even with a thrown weapon, it's often too valuable to throw away and the range doesn't stack up - it becomes more of a special case for melee than truly ranged.
No offense, period - Support-type characters tend to have this - no way to usefully contribute to a fight. They bring a pistol to a rifle fight, a knife to a gunfight, and an emergency backup weapon to a swirling melee. Since they can't do much, they end up doing nothing - foes can ignore them (or try to victimize them, to draw a reaction). This is often a side effect of the one-dimensional no-range-or-no-melee character design. "I'll just stand here."
Time is wasting!
This came up in my discussions of suicide by Great Haste. There is a pull towards maximizing what you do this turn.
It's possible to waste opportunities. It's possible to do too little and not contribute to victory. But the fallacy springs from thinking that you must do as much as possible each turn to create opportunities. This can lead to forcing ones that don't exist, or taking risks that aren't warranted to try to create a payoff. Just realizing that if there is nothing good to do, Waiting or some non-attacking move (taunting, inspiring friends with Leadership, getting into a better position, etc.) can be the best option available. Pushing too little is bad, but so is pushing too hard. It's this mistake that drives lots of running around the battlefield trying to attack the best target every time, breaking up formations, or doing costly attacks with little benefit to yourself or your friends.
Those are some of the combat mistakes I see pretty often in my games, and from reading other people's game reports.