I attended the first five minutes of a meeting thinking it was to discuss one thing - but it was to discuss something else which had nothing to do with my job. Luckily someone asked "Hey are we going to be discussing [stuff which has to do with my job function]? Oh - maybe LeonPowe doesn't need to be here then"
I realized it 30 seconds into the meeting - but since it was only 5 people I couldn't smoothly leave. I suppose I could've asked at first, but I was too dumb to realize how to get out of the meeting.