(the) next thing (one) knows
All of a sudden; out of nowhere; without warning. I was standing in line, minding my own business, when the next thing I know, a gunman runs into the bank and starts screaming at us to get on the ground! Tammy had only been on a few dates with Mark, but this one had been going smoothly. Next thing she knew, he got on one knee and proposed to her. I must have dozed off on the subway because I got on and, the next thing I knew, we were at the end of the line!
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