Store your cupcake liners incorrectly and they can easily become bent out of shape easily. It's probably happened to every baker out there, but there's an easy fix if you have a few round cutters at hand.
The Cake Blog passes on a tip from Lovely Cakes:
1 ¾" Round Cutter
2 ¼" Round Cutter
Step 1: Place the liners on top of the larger cutter and place the smaller cutter inside the liner and push it down.
Step 2: Let it sit for a few hours. Once you remove the liners from the cutters they will be perfect again!
Incidentally, one of the commenters on the post has a great tip to prevent your liners from bending. Lorie Andeson Caballero stores cupcake liners inside a small foam or plastic coffee cup, pushing them down just enough so that the accordions compress and stay tight.
Reshape Cupcake Liners [The Cake Blog]