The LulzBot TAZ 5 User Manual has this mysterious section:

5.4 Using 1.75mm filament

Your LulzBot TAZ 3D printer is set up to use 3mm plastic filament by default and may be capable of printing 1.75mm filament with no hardware modification. While many of our advanced users are able to do so, your results may vary.

Since we wanted to use the remaining glow-in-the-dark filament, we decided to try our luck. So we changed the filament diameter to 1.75mm in Slic3r, and printed a few small objects. Everything went as expected, only the retraction moves didn’t work so well and the filament oozed out of the extruder nonetheless. But those strands can easily be cleaned up later, so we went on with some bigger objects. This was when the printer would reproducibly stop extruding after a few layers, and it seemed like the nozzle was clogged up. Investigation showed this beauty:

Apparently the filament was too thin to be directed into the hotend, so it decided to curl up in the space between the hobbed bolt and the upper entry of the nozzle instead. After all, the manual was right, our results really did vary.