Deleted user and this error came up?

[pityocamptes]

A user wanted to be deleted, which I did, and I noticed that this error shows up on the header:

Warning: array_merge(): Argument #2 is not an array in ..../forum.php(791) : eval()'d code on line 60

I ran maintenance, and it seemed to have disappeared, but I was wondering if this is a more serious issue? How to fix? Thanks.

[Dave]

That's caused by a plugin which uses the forumhome_complete hook.
It's impossible for us to say which plugin caused it and if it's bad. Generally, it's no big deal if it only happened once.