One-handed amnesia refers to when a one-handed solver forgets an algorithm on a one-handed solve that the solver can normally do well using both hands. This occurs frequently because most cubers memorize algorithms with both of their hands rather than both or one.
One-handed amnesia is more likely to occur under pressure. One of the most notable examples of this was at the World Rubik's Games Championship 2003 on Chris Hardwick's second 3x3x3 One-Handed solve because of the pressure of being on stage after setting the then world record in his previous solve. One-handed amnesia still occurs in most, if not all, notable competitions to this day.
A notable oddity in the context of this phenomenon is Phil Yu who had "two-handed amnesia" where he would practise one-handed so much that he would forget algorithms when doing two-handed solves.