The on ERROR event triggers when an IRC Server sends an ERROR message, this usually occurs on a disconnection.
Format: on <level>:ERROR:<matchtext>:<commands>
Example: on 1:ERROR:*server full*:/echo Try another server!
For an explanation of matchtext see the on TEXT event.
on 1:ERROR:*banned*:/echo I am banned from this server *mumble*!
This triggers when you try to connect to a server and it tells you that you are banned. The $1- parameters refer to the full error message.
Note: This event is not related to any kind of error reporting in mIRC itself.