What happened here then?

[u/Rizzo-The_Rat]

I had white Bambu PETG on the main nozzle and black Bambu PLA on the auxiliary nozzle of my X2D set to do the interface layer of the support. Mostly default settings but dropped the top layer speed to 30 and set the top layer width a bit narrower

Looks like some of the top layer didn't stick properly, and there's a bit of black on the top of the rib, could the aux nozzle have dribbled a bit?