I was looking for errata on an old AD&D module, one I'm planning to run with my group in AD&D at some point. So I won't mention the module, or link to the discussion that I'm pulling quotes from. If you really want to know, search for the quoted text.
I've said it before, but I'm a big fan of actual play feedback. I'm less impressed with proclamations of what will happen than what did happen. Even an educated guess at likely results isn't the same as actual results.
Case in point, this module. It has an ambush right near the beginning. The module spends a couple of pages on the stats of the fighters, a mapped battleplan, and round-by-round discussion of ambush tactics. It admits right out that it's a high-level module and high-level PCs are likely to just open up a can of whupass on the ambushers and be done with it.*
I found a discussion of the module, with a really amusing example of what I mean:
Initial Post: "I don't know if my players are just good, but the ambush would never work. The PCs would know about it long before it was sprung and use their considerable resources to create a counter ambush [. . .] I know it's not that the PCs are overpowered - they're comparable to the pre-gen characters from the module (actually slightly lower level and ability scores). I know my players will [. . . blah blah . . .], use [blah], [blah], or [blah} . . . "
I got it. You players are highly skilled and won't fall for this kind of thing. This module is written for less sophisticated players, clearly.
After-first session Post: "We started play last night, aaaand...the PCs walked directly into the trap."
Turns out they figured out the ambush, but walked into it expecting to use magic, Charisma, and food bribes etc. to make friendly contact with enemies they'd been told were viciously evil man-eaters.
I'm not faulting the GM, here. I'm just using this as an example - projections and predictions aren't always accurate. "My players always . . . " meets "But for some reason, this time . . . "
Conversely, PCs can sometimes out-think a problem. Same module, another thread, someone pointed out his players deciphered the thing to do, and did it, and finished in a single session. Also something worth considering, and is Actual Play that shows a potential flaw in the module if your goal is a series of sessions and not an afternoon's quick jaunt.
* The module also has a weird couple of circular references to monster stats that contradict each other, so I was searching for an "official" answer to that but couldn't find anything.