I thought that I ran into an extruder issue yesterday evening. The filament simply got stuck. However, looking at the filament shown in the picture, you can see that the plastic thickens just before (to the left of) the spot where it got stuck and the extruder has eaten into it.
The issue was easily resolved by backing the filament, but the print in question was lost. This highlights the need of a sensor ensuring that the filament actually is moving. Perhaps something that can be constructed from an old mechanical mouse. Had I left the printer alone, it would had eaten its way through the filament, making the operation of backing the filament out of the extrusion head slightly more challenging.