Here are some little rules and rulings I think my players overlook. Feel free to use these, but double check the rules yourself. It's been a while since I did one of these for my gamers - but they're always well-received. I have more time I can justify spending reading rulebooks and keeping track of rules that affect all characters and not just one or two, but my players mostly do not.
Intimidation skill is improved by displays of strength or violence (+1 to +4), but isn't based on ST, it's based on Will. Being big and strong can't hurt, but it's not automatically going to back people down. (p. B202)
Leadership hasn't had a ST default since 3rd edition. (p. B204)
Two Handed Grapples of the same target location is just a standard attack; you only need multiple attacks if you want to grab two different locations on the same turn or want to make two one-handed grapples. In fact you can use any number of your arms, and claim a bonus for each arm after the first. (Grappling p. B370)
Swarms don't have to roll to hit you, nor can you defend. They are animate area effects, basically. The only way to "dodge" a swarm is to get out of a hex it can reach. Armor usually protects in some fashion, but it depends on the swarm type. (Swarm Attacks, p. B461)
Swarms are vulnerable to shield bashes, stomps, area attacks, and normal weapons differently, depending on the swarm. (Swarm Attacks, p. B461)
Low Mana does affect skill for purposes of determining a spell's energy cost. (Energy Cost, Magic p. 8)
Spells resist countering spells based on the caster's effective skill when cast, not base skill. Don't cast that Scryguard or Magelock when you have 10 spells up! (Resisted Spells, Magic, p. 14)
Those are ones I've seen come up recently. I know at least one player at my table is aware of each one, but I'm not sure all of them are familiar with all of them.