Interference is the prime cause of forgetting. New knowledge easily displaces old knowledge in a process that may result in minimal learning. Reducing interference may be an important component of building memory stability as predicted by the Neurostatistical Model of Memory. Supermemo 
Interference is good if you're trying to displace old knowledge. However, if your goal is to retain stable memories of information important to your learning then this may be bad.
Retroactive interference: The process of overwriting old memories with new memories.
Proactive Interference: Interference will also cause difficulty in forming new memories when the stability of old memories is high.
See also Flash Card Patterns.