Bugs are programming errors or "glitches".
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.
Occasionally two vultures may lay a spidermine on top of each other. When this happens, the two spidermines cannot move to attack enemy units. Instead, they will unbury themselves, and stay in their position until they are destroyed.