Underestimate your opponent. Just ask Cub Swanson, who's talking about his loss to Jens Pulver in the WEC:
When he put the choke on, I thought it was just a guillotine. You know, I'm facing the ground, I can't see anything. So my first reaction was `OK, no big deal. I'll pull guard, my head will slip out, and we'll still be on the ground in my world.' You know, I said `he doesn't want to be there.`
So when I went to pull guard, my neck cranked pretty severe, and I realized it was a wrestler's front head lock. We would call it like a no-arm D'Arce, or a no-arm anaconda, but it's really a wrestling move, and that's the only reason that he knew it. So I would have had to fight the hand instead of pull guard. I guessed the wrong choke, and all I did was make it worse.