There are a whole lot of mistakes I see people make in the various arenas, and there are several that many of the more "experienced" pilots count on people to make and exploit them. I myself make some of these "mistakes" all the time, simply because I count on most people in the arenas to not know how to take advantage; mind you, whenever I come across a decent pilot I'm screwed.
Here they are, in no particular order:
1) Attempting to out-turn a slower con. This is one of the most common mistakes among the newer TnBers (spits, nikis, etc). It doesn't really matter what the other guy is in, if you're closing on him fast, he's going to turn tighter than you, and if you go after him, you're screwed. If he turns, go into a zoom climb or something. This is in fact the most common mistake I come across.
2) Making a few turns, seeing that the other guy knows what he's doing, and giving up. Quite often I'll be attacked by someone in a spit and go into a rolling scissor, or make a few turns and try to rope them, and all of a sudden they'll give up and try to get away. Now that CAN work, but it depends entirely on what you're in and what the other guy's in. If the other guy can accelerate better than you, giving up and heading for home is one of the worst things you can possibly do.
3) Waiting too long to break when being bounced. Quite often I'll dive down on a con, get in fairly close (around d800-1k), and then the guy will break left, right, or simply attempt to do a loop. This isn't a great idea; if the guy bouncing you is anything approaching a good shot, you're going to be smacked pretty hard. When a guy is in that close, he'll have enough time to adjust and get his nose on you. Granted it will most likely be a shot with 70-90 degrees of deflection, but I find deflection shots easier than shooting from dead 6. If you want to wait until the last second to break, for gods sake, do a split S (looping towards the ground).
4) Holding hard turns for too long. This actually happened the last sortie I flew. I was fighting a Spit IX in my 109G2 for 30 seconds or so when he went into a hard, flat sustained turn. Myself being cocky and a complete sucker for that kind of thing, I followed him. I couldn't keep up, and if the guy in the Spit (forgotten his name) looped towards me or something I'd have been screwed, but he just kept turning, and I stayed with it. The Spit was turning a hell of alot faster than me, but eventually, I got my nose on him and hit the 90 degree deflection shot. This happens alot; people assume that the plane with the larger turn radius will just squirt out in front of them without putting themselves in danger, but that just isn't so. Again, any good shot will likely hit that deflection shot and rip through you before he squirts in front.
I'm having a brain fart at the moment and I'm tired of typing, so I'll leave it at that. If I think of anything else I'll add it, but if anyone else can help me out and put some things I've missed, go for it.