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When you accidentally manage to get in someone else's photo. (And it ends up biting you in the ass later. i.e. Getting caught cheating on your partner or otherwise lying to them about where you were and/or who you were with.
Me: "I was looking at cool event photos from a few years ago, and saw my husbands lying ass drinking with a buddy and two chicks. He said his ass was going for a ride on his motorcycle that day!!"

Friend: "Unintentional photobomb... Does it every time."
by Sporadiclove February 28, 2015
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