I went to a cosplay fesival here in Bangkok. (amazing colors, I love them) I am really, really careful with my gear and constantly aware of where everything is, both outside objects an my camera. The lens (24-105) had the hood on as well.
Long story short, I came home at the end of 3 or 4 hours of shooting and I had a smudge on the filter. No big deal. I took off the filter and tried to clean it, no luck. Basically I have no idea what it is, but I can't clean it with any method or cleaning solution. Throughout the day I never bumped into anything.
I know the front elements are tough, but I still wonder what would have happened without a filter.