When tanning Superior skins, if the quality drops, within one task, it seems to always drop 2 levels to Decent.
As working on Fine skins, it doesn't ever seem to drop to Harsh; which leads me to believe the 2 level drop could be a bug.
(Fine broad knife, Hideworking 82)
Hereby just confirming this rather old report as not being a bug.
There can be 1 level drop with tanning as well, but due to how the math is handled it's not often seen, and moreover the character's skills based craft quality
greatly here so the same character tends to see the same kind of results. Tanning process is quite forgiving here in a sense so that serious quality degrade isn't even
possible anymore at the last stage of tanning. And even for the quality to drop at all here the tanning bad performance needs to fail quite badly, thus two level drops
are often seen. If the tanning would fail only to small degree the system works for the character's favour and no quality drop is registered but the original quality is preserved.