Avril Lavigne's Ex-Husband -- Taking Her Last Name Was a Huge Mistake

Here's something you didn't know about Sum 41 singer Deryck Whibley -- he took Avril Lavigne's last name when he married her, and now ... 3 years after divorcing her he's finally sick of it ... because he's filed legal docs to drop it.

Deryck Jason Lavigne Whibley just filed in L.A. County Court to go back to his birth name ... sans Lavigne.

It's pretty crazy -- A) because he took her last name in the first place and NO ONE KNEW, and B) because it took him so long (FOUR YEARS) to switch it back.

Whibley and Lavigne got hitched in 2006 and divorced in 2010.

To review: Deryck ended up stuck with Avril's last name -- and Avril ended up ... stuck with Chad Kroger.

It's a toss up who got the worst end of that deal.