Problems with boolean subtract, or removing materal

[jleslie]

I keep having problems with this. with simple 3d objects there isn't a problem. but
if you make a polygon, or try and boolean -> subtract a couple of times, the program starts to fail.

Here is a simple demonstration. I've made a trapezoidal extrusion, and I want to remove that shape from a simple cube. boolean -> subtract does weird things.

Any suggestions?