They may be caused by many things, the most common being different path lengths in a circuit. Experiments 1 and 4 show examples of circuits with glitches. To prevent glitches, remember a few basic rules. In synchronous circuits, make all transitions on correct clock edges, usually the same edge for all transitions. In asynchronous circuits, make sure that only one input changes at a time.
Glitches are often difficult to detect, since they only last a few seconds.
If the static situation is correct before and after a state transition, but the state transition is incorrect, or a counter or shift register advances more than once, a glitch is a likely cause. Use a timing diagram to discover a glitch if you think they are present. Occasionally, but not often, equipment in the 249 lab will fail. Don't be too eager to blame the equipment though. Most cases where students blame the equipment are caused by other things.