Thunderbird oddity

I had a minor but really annoying issue with Thunderbird: every time I opened a message, it would search for the first occurence of "no" and select that. This meant that if that "no" was in the middle of the message I needed to scroll to top of message.

In the end the reason was simple enough, I had a quick filter set at some point (Ctrl-F), but I can't think of any other application that works quite this way (remembers the setting & applies it automatically).