Me, oftentimes, since I tend to get too railroady sometimes and use DMPCs to fill the gaps in smaller groups (1 at a time, of course).

But my all time best on the DM red flags I ever experienced was a DM that brought a Frenzied Berserker DMPC and never ever fudged rolls for the sake of... well, anything. No malicious intent, he just didn't see the TPK coming.