I've bastardised patterns, extrapolated patterns, and thrown patterns out the window in the course of cosplay. (To be honest, though I'd sewn garments for almost ten years when I started cosplaying, I'd never actually picked up a pattern before. My mother laughed at me when I came to her for help, and then got very confused when I started changing things.)
I turned the shirt I used for my Road Kamelot cosplay into the pattern for my Miranda dress -- and I'll probably be bastardising another shirt for my Kaya cosplay. My Risai? Two different bastardisations of the same chinese coat pattern, and a lot of "hmm, this should work!". Patterns? More like guidelines.
Really, it's kind of fun, though.