So, I accidentally threw out my co-worker's lunch today at work.
That happens sometimes.  Was it mistaken for something that might have been sitting there forever?
Yeah, it was when I was gathering up my trash when I was wrapping up my lunch, so it was an honest mistake. I felt pretty awful about it.
I'm sure if you explain it to them, and maybe treat them to lunch another day, they'll forgive you, if they haven't already.