Tangled
A tangled, a later part of the filament is over a newer part of the filament, filament is rare from the manufacturer. It does happen when something goes wrong in the manufacturing process.
By far the most common problem is user error. The end of the filament should be in your hand, in the extruder, or in a clip on the spool. Keep this up and you will rarely have a tangle.
I’ve got a spool and a bunch of filament came off, what do I do. Pick a spot on the filament that is still on the spool. Take off all the filament before this. Make sure there are no tangles in the part that is off the spool. Hold the end. Rewind the filament. Don’t let go. If you come to a point in the respool where your arm is under the filament and you can’t get it out without letting go of the end, you did not make sure there are no tangles.
The filament wants to hold it’s shape and spring back. If you let go of the end, it will tangle, it hates you.