When I use zsh in interactive mode, I get some glitches. This mainly happens when the command spills over onto a new line and I use backspace, with backspace leaving behind some glitches on the screen and moving the cursor to an odd position.
It happens in a VT, in xterm and urxvt, although it is most noticeable with my chosen terminal, urxvt.
When I use zsh as a login shell, it does not happen at all.
What could be causing this?