Trying to copy pixels within the video buffer.
If I put memset(0x10000+x+w*y,7,1) and memset(0x10000+x+w*refl,10,1) in my code, the expected pixels get set and nothing strange happens, but if I try to do memcpy(0x10000+x+w*y, 0x10000+x+w*refl, 1) then Picotron locks up completely - the music keeps playing but the program is unresponsive to the point that Windows gives a "not responding" crash popup.

Fixed in 0.1.1d -- thanks for the report @NMcCoy
It was going into an infinite loop when len < 4 (!)