As far as I've read on multiple forums, batches manufactured in August, September, November, January included a few faulty units, whereas the ones manufactured in October, December and February were alright.
But don't take my word for it, this is simply my conclusion based on various threads from various forums.
Sent from Bacon
Meh, there's not much I can do about it anyway. The biggest issue is the vibrating motor - which can't even be changed with one from another phone (say, a Galaxy Note). OnePlus doesn't put out replacement parts neither.