Using mod podge to apply a piece of 8.5 x 11 paper to some chipboard, and having it warp and wrinkle into near-unusability.
Using too much glue is easy to do. Ideally you want to apply a very light layer that's within seconds of drying. Rolling your paper down gradually while applying with a foam brush works for me.
I made an oops while binding a rules booklet. Never use binder clips to hold down freshly glued paper binding. Nasty mess.