Basic blender went bad (motor ran but spindle wasn’t rotating). I wanted to disassemble to see if it could be repaired. Three of the four screws were Phillips head. I had to cut the casing open in order to discover why I couldn’t unscrew the fourth. It was a slotted spanner.
I think this was at the bottom of a deep hole, as you can see where the plastic was cut around it. A standard bit and holder wouldn’t fit down that, I don’t think. What the other guys said about a flathead and dremel/grinding wheel is the only option really, but you’d have to be able to ID the little fucker first.