The customer support efforts when you don't feel like it, being ghosted after helping a customer, the random or fraud disputes.
It's really tricky at that stage between hiring help and having the time/motivation to maintain those very non-tech parts while trying to continue doing other core parts of the side project / startup.
The first sale feels great, as does first showing the prototype.
By comparison, extra $100 MRR milestones don't feel so great, nor does dealing with customers/disputes eventually (it's a lot of negativity in general - pleased customers just leave reviews occasionally, negative ones email you). And a down negative month or two always feels like a stabbing and like it's all over.
Really don't know how to avoid this. Scaling quickly? Via investment in most cases? Maybe.