At FL 380, OAT -65ºC (I think), something that probably turned out to be water started gushing from the overhead bins, on both sides of the aircraft (outboard seats), around row 23 (just aft of the wing-root leading edge).
I would not expect this at the above mentioned phase of flight, most commonly on pushback or rotation. We had an early flight out of Miami in a 742 once a week, middle of the summer extremely high humidity, we would taxi the aircraft from remote parking to the gate leave the APU running and install external A/C to augment the APU air and obtain a a decent cabin temp. We had several gate returns due to passengers flipping out on taxi out.
The level of humidity at altitude is about 0%, possibly trapped condensation that drains after level off before being reabsorbed into the dry air, I can not think of any other possible answer.