Bug

Bugs are programming errors or "glitches".

StarCraft
A bug prevents the Valkyrie from attacking if there are too many units on the screen.

A "crystal archon" is an archon or dark archon with a permanent stasis crystal animation from the arbiter stasis ability surrounding it. The creation of crystal archons is possible due to a glitch, and can be done in StarCraft by using stasis on a forming archon just before it is completed. A crystal archon is no different from a normal archon besides its appearance.