Review (4/5): Reason for leakage during flights ...
The bottles all contain some air, which will be at ground-level air pressure. The pressure inside the jet, at altitude, is lower - so the air inside the bottles expands. If not sealed tightly enough, it will leak and take some of the liquid with it. Best thing is to squeeze the excess air out before traveling. Since these are a hard plastic rather than a readily squeezable version, it is recommend that you transfer into squeezable travel containers. Understandably, for me, anyway, this negates the use of these in the travel size. I buy full-size and portion out into flight-friendly containers.
Hope this helps.