Doing an alteration to a pair of trousers, I changed color of tread, loaded the matching bobbin, and the proper size needle, then proceeded to sew.
Perfect seam except for a 2-inch long snarl in the middle of the run. Everything was fine before I made those three changes, so it had to be something about them. Thinking about it the thread was old, the bobbin had been sitting around forever, and the #14 needles had come with the machine (my old Singer 750) when I bought it well used from the Good Will store several years back.
Rule one: never put a used needle in the box with new ones. But did the previous owner go by that? No way of telling, so off to the store for some new #14’s.
Just how old was that thread? I do not remember buying that color myself. Maybe it came with the machine too. Picked up a spool of that color when I bought the needles.
How long had that bobbin been sitting the the accessory box? No telling, better strip the thread from it and wind some of that new thread on it. While stripping the thread it hung up a few times.
That most likely was causing the snarl, sudden high tension on the bottom thread.
Ran a long test seam, no problems.
The point here is to think the problem through and try the most likely causes first, rather than just trying things willy-nilly.