My achilles is long term contracts. Every time I give out a big deal the player seems to get injured or regress tremendously; best two examples:
Hand out a new 5 year, 100 million dollar deal? 60 day DL by the end of the first season after never being out for more than 3 weeks his whole career.
Give star player and team leader a nice new 98 million dollar deal? Tears his ACL in July - out 8 months. At least the back-up randomly became a stud hitting 30 homers. No way am I giving him an extension though; been burned 1 too many times