I'm using thunderbird. It just happened that I clicked some keys, and now thunderbird is hiding several mails from me.
Strangely, it still shows them to be found when i search for them (in the examples below, it shows 5 or 21 results, respectivly; but only 1 or 0 mails are displayed).
I think it has something to do with "Themenbäume" (dont know the english word), because just after I hit those "magic keys", there has been a strange sign in the "Themenbäume"-row.
I was searching google, but didn't find similar problems yet.