Also, a lot of blame is directed at Jason Allen, but may I remind you...Yeremiah Bell was awful last night as well.
I can only guess that you've never played sports a at a high level. These guys are professional athletes, and at that level, slipping is more often than not a function of balance, positioning, and general athleticism. It happens, but when it does, it's on that player. On that play in particular, it was clear that Allen was caught way out of position, and he slipped trying to make an extreme change of direction in an effort to make what should have been an easy tackle. Moreover, knowing that he had no deep help, he should have factored that in and made sure not to get in a situation where a slip (or missed tackle for that matter) would result in a touchdown. Better to give up a 20 yard completion and then force the WR out of bounds than to do what he did. If we're talking pee wee sports, then sure, slipping happens and you can't blame it on the player. But at the NFL level, slipping is never an excuse, especially when it causes a long touchdown.