When someone wants to change his (or her) avatar on MyBB 1.2.3 the user himself doesn't get any error messages when the size (or something else) of that avatar is too big.
That same issue that you have mentioned
here actually. I have explained there why this is happening.
And it is a bug in displaying all the avatar error messages.
regards
Err you need to update your templates while updating your MyBB.
Someone screwed up the templates.. *innocent*