Last weekend (Nov 2 & 4) my son and I went ORD-MSP-ORD. Before the fist flight my son asked me what would happen if everyone is early (we were at the gate 90+ min before flight time). I told him that they flight might take off early, but in 25 years of flying it happened to me only once, so our flight will not take off early.

Sure enough, both flight took off about 10 min early.

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Sagy