I noticed this when I disconnected the Internet cable from my machine
and then plugged it back in a while later to reconnect.  When I hit C-c
C-n to list the users, a bot of mine was listed among the users.  I did
/whois the_nickname and got "No such nick/channel".

After some investigation, I found that ERC doesn't clear
erc-channel-users when an unexpected disconnection occurs.  I've
attached a patch which addresses this problem.