If the "Never Ending Story" is any guide, someone needs to start reading right now.

It's basic science, really. If a plane takes off in very hot, humid weather at low altitude, then as it climbs and the cabin pressure drops and the air cools, there's a lot of excess moisture that can spontaneously form into a cloud. It's not a danger to anyone (except if you're afraid of tripping and falling in the aisle), but it doesn't make you feel like you're not in an episode of the Twilight Zone. What is perhaps the craziest thing about this is that photos of this phenomenon were posted not once, but twice to reddit within 12 hours of each other, on what appear to be different planes (and a quick Google Images search indicates the photos themselves are new to the Web). 

Sources: redditor 2bsahm1 | redditor harrisonz08