walk in on
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walk in on (someone or something)
To intrude accidentally on someone or something during a private situation. I was so embarrassed when I walked in on my roommate and his girlfriend the other evening. I think I walked in on a conversation that I wasn't supposed to hear.
walk in on someone or something
to interrupt someone or something by entering a place. I didn't mean to walk in on you. I didn't know anyone was in here. Alice walked in on the meeting by accident.