A: It's when your past self forgets to buy milk, but your future self still has to deal with the consequences.
See also: Predestination ProblemsA: Call in a team of highly trained temporal engineers, or try bribing your past self with a nice bottle of wine.
Timey McTimey: Temporal Anomaly Repair ServicesA: Unfortunately, no. But have you tried asking your great-grandfather for help?
Predestination Problems