Wikipedia:Hindsight

I bet you've had a disagreement with an editor and decided to dig a bit into their contribution history before. You're digging away when you finally find something juicy! It's an old action/comment the editor made in "good faith" but looking back on it now it turns out it was a really bad move by them.

It seems a lot of editors here on Wikipedia see that as a bit of a goldmine in an argument, and won't waste any time to drag it up to 'prove' how awful that person is and that their points are now invalid.

If you're tempted..
If you're tempted to bring up an old transgression or mistake of an editor, remember back then in that situation it may have seemed like the best idea. You weren't there, nor were you having to deal with the issue in the same context as that editor. It's always best to forget and forgive previous mistakes, and focus on current ones.

There are exceptions
This doesn't always apply though, for example if an editor's actions were especially stupid or clearly weren't done in Good Faith, then by all means drag that up and rub their faces in it (though be civil yeah?).