I had a wiki page with some pre-formatted text (a text visualization of a tree of things). The first line had a bunch of leading space followed by the text for the root of the tree. Anytime I edited that page, the leading space would be removed (even though I didn't edit that line). I'd have to always remember to go to the wiki markup and insert the leading spaces again. The following lines also had leading spaces, but those were preserved as expected.

I've heard from colleagues that this is not limited to pre-formatted text; it happens with the first line after many other confluence tags as well. I haven't verified this, but it's probably true.