Funnily enough a few months ago, I discovered a small moth inside my 24-105L lens. How and why it got in there I don't know, but its body was loose inside and clearly visible. The lens was old and I wasn't going to sell it, so I decided to open it up to remove the moth and clean it. But first I took some shots to see what effect it had, and I couldn't detect much, and decided that I was likely to make the lens worse by fiddling with it, so I didn't bother. I've been using it ever since and can't say the images are any worse than before.