If an multi-block advanced monitor (at least 2x2) is constructed facing downwards, with its "topmost" edge pointing east, then its northwest (or "bottom rightmost") corner block will glitch out.

Fixed for 1.79. I had to build some fun test structures to test every single monitor orientation!

Heh, I did forget to test the upwards-facing displays more thoroughly. I hope you linked them up as disco lights afterwards! ;)