I voted the munchkinism part, though really I consider it more of "optimization". Out of our current circle of players
I've been playing 3rd edition since it came out, and DMing about once a week.
One has played d20 a fair amount, and understands the rules well, and has been in my campaigns for years.
One hasnt played d20 before this, though has read the rulebooks and has a fairly good understanding of the game.
Two have never played before, but have a moderate understanding of the mechanics their characters use frequently.
One has no clue.
Most of us have been playing about 20 years, they just havent been in a 3rd edition game before. As such, they tend to randomly pick skills and feats... so I tend to nudge them in the right direction. One player thought she was good at disarming traps... we looked at her sheet one section, and she had 2 ranks at level 7, so I explained that disable device is one of those skills thats more useful if its at the max rank for your level (and that a +3 on disable device is really just going to get you into trouble). Other players took skill focus in skills they didnt have ranks in, etc. I've found the best way for them to survive is for them to describe a concept, and we try and find some rules that make it playable. I really hate games that are fudged for or against the players, so needed to nudge them a tad towards the "useful" end of the spectrum. This has helped keep the 2 players who understand d20 tactics from being bored, and the 3 newer players are catching up fast.
So I REALLY dont have to worry about anyone intentionally breaking the system. I have to worry more about people using weaposn they are proficient in, remembering that when I call for an attribute check they no longer try and roll under, etc.