This doesn’t happen to me all that much, but I see what you mean. What I’d recommend is marking a 2-hour block (say) on your calendar for every afternoon (say) that says “unexpected things”. You may not be able to predict exactly when these unexpected things will happen, but this block will (1) make sure the number of things you schedule for a day account for these unexpected things and you don’t plan for too much, and (2) give you flexibility to move things around in the day depending on when the unexpected things actually happen. Does this make sense?

It does require you to re-plan, and I do re-plan often (sometimes more than once during the day).