It is a coincidence. My 650 fine. I've not read of problem of 650s locking up/freezing. Besides, I've read of problems with all readers and that does not mean an inherent flaw.
I did have to do a reset on a 505 tonight. But that was partly my fault. A defective eBook (missing a >) was causing a chapter not to display and I trying to jump to a page within the missing chapter and the 505 froze. Plugging it into the computer did not get it to respond. But I reset it and it was fine after that.