Forgetting you owe someone money is a memory problem you only seem to have with debts.

Memory is suspiciously selective about obligations. Forgetting is genuine often enough, and the pattern is still worth noticing, since the same mind that loses track of what it owes keeps careful records of what it is owed. That asymmetry is rarely deliberate. It is completely visible to the other person, who has certainly not forgotten.

Write down what you owe the moment it happens, including the small amounts. Pay it back before the other person has to work out how to ask. Few things cost a friendship faster for less reason.