If the answer is not community wiki, which invites such edits, then I reject them. The usual reject reason I use is "radical change".
A suggested edit I just saw in the queue makes a great example. This was completely new content that was different and unrelated to the existing answer, and should have been posted as its own answer.
The person who originally posted the answer may have had a reason for not including the content, other than not knowing about it. Such edits cause the user's answer to be something significantly different than what the user intended.
There are two circumstances in which I might approve such an edit:
- If the answer is CW, as I already mentioned.
- If the suggested edit is obviously posted by the same user, but from a different account. (It's not terribly uncommon for new users to get confused and create multiple accounts.)
