Revealing mistake: When the first tank comes during the final battle, look carefully at the German sitting on it and you will see that he's actually a dummy.
Revealing mistake: When trying to take the warehouse, we see a German machine gun position in a lower window. Pay close attention to the brickwork below the window. There are several marks visible that all end up with a bullet hole in the middle of them, hiding the charge for the effect. (00:14:30)
Answer: To mess with him. A lot of the "regular" soldiers didn't respect or even like Webster early on because he was a college boy. That dislike and lack of respect was deepened when Webster took what they perceived to be an extended stay in hospital after being wounded when many of their other comrades - Popeye for example - left hospital early to get back with their friends to fight.
He couldn't leave hospital early because he was in England. The wound in Band of Brothers was perceived a lot less serious than it was, and a lot of easy men were happy to have him back, especially Luz.
If you read the literature around Band of Brothers, you'll find that Kenyon Webster was perceived as a slacker in the sense that, while not precisely a coward, would never volunteer to put himself in harm's way. He only did as told and nothing more. So, regardless of the reasons why he really couldn't go back to Easy, his friends were predisposed to feel he was slacking off at the hospital while they were risking their lives.
Nauticalisimo