What payment challenge creates the biggest problems when a business starts scaling?

A lot of businesses focus heavily on getting more customers and increasing sales, but payment operations often become a problem later.

Things like failed transactions, changing customer expectations, managing different payment methods, or keeping processes organized can become bigger challenges as volume grows.

Curious to know from people here:

What payment-related issue caused the most trouble for your business while scaling?

Was it something you expected, or something you only noticed after growth?