I had an incident recently where another user decided my answer was wrong, and simply edited my answer to change it, and commented on the question that it was previously wrong, without any explanation why. Because there was no comment on my answer, I didn't get any message to indicate what had happened, I only noticed by chance.
Frankly, I feel it's more than a little inappropriate to change the substance of someone else's answer without any prior discussion; am I right in thinking the user in question should have commented on what the problem was in his opinion, and/or posted his own answer rather than simply changing mine? I can probably see why he felt it was wrong, but depending on the context of the OP's question, my original answer could well have been exactly what he was looking for.
Hence my question; I've edited questions myself purely for formatting or spelling, but never to alter the substance of the content, but aside from making a question/answer easier to read, I can't see any other justification for it. Is it poor etiquette to modify another user's answers in this way, especially without qualifying your reasons?