Our Prusa Mk4 only has one extruder and no MMU. But I was playing around with filament change in the middle of a print to have several colors in a single part.

So I drew a small part with 3 characters connected by a common "bridge". Printing the characters face down, I tried inserting a color change at the first layer of the bridge in PrusaSlicer:

This works fine: the printer parks the head in the lower right corner of the bed, ejects the filament and wait for a new filament to be fed. Then it purges for a while and somehow manages to cut the purged filament clean and continue printing. A bit sketchy but it works.

Then I tried a 5-color print with multiple colors in X, Y and Z. Obviously the filament needs to be changed mid-layer for most of the layers. I defined 5 virtual extruders in PrusaSlicer, added a custom tool change G-Code to force a filament change with M600. After a bit of messing around with layer thicknesses, I managed to limit the print to "only" 16 manual filament changes. A bit annoying but it's okay for an experiment:

When I launched that print, it started out well, printing the first layer of the green "5" digit.

Then it hit the first M600 instruction, proceeded to eject the filament, and horrible noises from the extruder ensued:

https://toobnix.org/w/5M5n1ZxxPjHgp9uPWUKu8s

I could NOT free up the filament for the life of me, and the printer wouldn't come out of the menu to resolve the problem. Eventually I had to reset the printer and apply force to finally clear the problem.

Fortunately, no damage occurred and I could reload new filament as if nothing had happened. I thought it had been a bad fluke, so I restarted the print and it did it again.

Before I file a bug, anybody knows if I'm doing something wrong here?

The printer is running the latest firmware as of today - v6.1.3, released in September 2024.