As waterrockets mentioned, if recovery between anaerobic intervals is neglected, the intervals will quickly become more aerobic than anaerobic. Sufficient time must be allowed for anaerobic capacity to be replenished. Don't have the exact numbers in front of me, but I think one is about 90% recovered after 2-3 minutes, and fully recovered after 4-5 minutes.

This is one reason why some advocate not shortening the rest periods too much during anaerobic intervals. Even though it is more akin to race situations, where recovery may be limited, the goal of the workout is ideally to train your anaerobic capacity. With limited rest between workouts, the intervals may hurt, but they will be much less anaerobic than one might suspect.