Always keep an eye on your feet and where they walk

Industrial feet, feed, and needle plate

Aside from tension issues, one of the most often found problems with industrial sewing machines is keeping an eye on your feet (walking or otherwise), your feed dog(s), and your needle plate. 

FEET- 
So, you'll always expect your feet to be in the proper place because you likely never touch them aside from changing them out. However, something that moves a ton and is right beside them is the needle. Always be sure to keep an eye around the needle hole, to make sure that the needle hasn't put a mar in the edges, or dug its tip straight into the side of the foot. This is especially problematic for welt feet machines, because one side of the foot is about the thickness of two pieces of mechanical pencil lead stacked on one another. A heavy duty needle can punch through that faster than you can blink. This will often cause the upper thread to fray out or cut randomly and for no apparent reason.

​Always keep an eye on them, especially if adjusting the tension isn't quite cutting it. 
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FEED DOG- 
Feed dogs are really difficult to damage, the biggest thing to look out for on them is to make sure they are coming up high enough to grip the fabric, but also dropping down enough to release it when the needle is in the fabric. This is part of timing a machine, and usually means you need a hand, but it is something you can check on your own. 
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NEEDLE PLATE- 
Here's one of the places that some "unexplained" issues can be resolved. If you're sewing a good thickness of fabric, or a dense fabric, or even stretchy fabric, it is possible for your needle to move out of alignment during sewing and manage to hit the needle plate. A full on needle point hit will almost always break the needle, but if it just glances off, it will only leave a mar in the needle hole on the plate. This can catch on the upper and lower threads, and often effect how your stitch looks, or either of the threads could fray out and tear randomly.