It is unlikely that you would "forget" the landing speed of an aircraft you have been flying, but there are occasions you might not KNOW the landing speed - I flew an Isaacs Fury, the person who had flown it before had crashed it twice, the first thing I did once airborne was climb to height and stall it, only to find the speed he had written down for approach speed was 2 kts above the indicated stall speed, there have been a couple of other times I have had to do this as well, but never because I have forgotten a known speed.