Known bugs
Markdown editor hooks (e.g. mse224533, math11392, boardgames1152, french347) sometimes fail to run on Opera, apparently due to a race condition where the editor gets initialized before SOUP has a chance to add the hook. Reloading the edit page usually fixes the problem.
The kluge used to monitor WebSocket messages on SE chat doesn't seem to work on Opera, so SOUP falls back to polling for changes at ½ second intervals. This works, but is less efficient.
SOUP does not work (at all) on Scriptish 0.1.11, because document.documentElement is not available at document-start. Apparently, this is a bug in Scriptish that has been fixed in Scriptish 0.1.12, but that version was never properly approved for distribution on addons.mozilla.org.
The new mse154788 / stats1987 fix in SOUP v1.22 may sometimes add spurious scroll bars to comments. Alas, I have not been able to reproduce this bug; any help in reproducing and diagnosing it is welcome.
Bug reports
If you've found a bug in your SOUP, please post a comment below to notify me about it.
This includes any unwanted side effects or regressions due to changes in the SE codebase.