I was on http://www.letsrun.com/ and I found a pretty funny thread. I have heard many of these and have even given some of these myself. The funny thing is even after giving some of these LAME excuses the person will often times run GREAT! Here are some of the responses I've noticed
10) "This is only a training run" or "I'm going to tempo this race."
9) "I haven't done any speed work so I'm not expecting much."
8) "I just decided to run this race 11 o'clock last night."
7) "I may not finish the race as I'm having bad shin splints."
6) "I only got 2 hours sleep. I was out drinking last night."
5) "It's too hot/cold/windy/rainy/sunny to run today"
4) "I'm not sure how I'll do today. I'm too well rested."
3) It's an 8am race start and the person says "You don't have to worry about me, I already ran 16 miles this morning."
2) "I had 2 BM's this morning. I wish I had a 3rd one."
AND THE NUMBER ONE EXCUSE HEARD BEFORE A RACE, drum roll please...
1) "I just got out of jail 30 minutes ago, long night. I'm not expecting much."
Track meet #3. 400 in 58".
13 hours ago