BTW - I've often had things going along fine and then BAM - thread breaks and I can't get it to sew. And it's always been something strange that the thread looped around or popped out of or something like that.
And sometimes I've had thread with bad spots in the middle of the spool - I've stripped off several yards of thread until it looked better, then gone back to sewing without problems. But that usually showed up as thread suddenly fraying at the needle. And rarely, I've given up and tossed out a spool because it just wasn't working.