If you use the markdown editor and include some raw HTML tags (e.g. br tag for table formatting), then edit the post using the rich text editor, the HTML content is lost. I don’t know if this is the case for all allowed HTML tags, but at the very least it happens for br tags. This may not be a big deal for regular posts since most users presumably stick with one type of editor, but for wiki posts this is a major issue. Over on WaniKani, we have a massive wiki post with several tables that rely on br tags for structure. However, if someone happens to edit the wiki in rich text mode, all br tags are removed, ruining the formatting of the post.
(Secondarily, there doesn’t seem to be an easy way to revert to a previous version of the wiki. If that was available, at least some of this pain would be mitigated. I can write up a feature request for that as well if the feature doesn’t already exist.)
Markdown editor:
Rich text editor:

