Not defensive ammo, but I've been using mini-mags for years to plink.
Today I had a box of the stuff in which about every 6th round had a lump of a soft waxy material built up on the side of each bullet.
Didn't notice the wax at first, but what would happen is the ammo fed ok, but then wouldn't extract. When you looked in, you would see this foreign waxy-paperish looking stuff on the ramp.
I had to peel the wax off some of the bullets with my finger nail before loading the magazine. Once done, everything worked fine.
I've never had anything gunky like that from a brand name manufacturer's stuff.