Well, Willie Made Me Laugh
When Julia Hoffman held back on Barnabas' serum and it caused him to revert, he also could no longer see his mirrored image.
When Barnabas is getting ready for the costume party, Willie helps him on with the cape.
"too bad you can't see your reflection anymore, Mr. Barnabas, you look great!"
I had to laugh. For quite a while too.