Minibuffer bug

I recently experienced a bug I can't reproduce: I somehow managed to use the write-file command on the minibuffer, and didn't notice until after it got saved. Subsequently, the buffer kept coming up first in the list whenever I did an iswitchb-buffer, until I renamed it to '*minibuffer*'. Does anyone have any idea how this could have happened? I can only think that I had some pending command that required input in the minibuffer, didn't notice, and tried to do a write-file while input was there. But when I try to do that with a new session, it just whines "Command attempted to use minibuffer while in minibuffer", as it should.