If you've been to a movie double feature, you know that you'll have a hard time remembering the plot and details of the first movie immediately after seeing the second. this can't be explained by the greater amount of time since viewing the first - infact, your memory of the second movie remains stronger a couple of hours later.
This fenomenon is called rectoactive inhibition or inference, and it has demonstrated to apply to a wide variety of learning and memory task, with greater similarity between the first and the second task correlating with a greater degree of forgetting. Knowledge of this fenomenon has led to a view of interference of one body of material with previously- learnned material as a fundamental cause of forgetting.