In my case, I had a client try to pay me 10+ times in one day because they thought it was their fault.

After I wait for support and get transferred, and then answer security questions two times, they tell me i reached the monthly invoice limit (it was $5k). 

Instead of saying "This company has reached their invoice limit, you will need to pay them after X date" or literally any other more helpful message, they were getting told to try again as if it was an issue on their side. And then following this post they "cleared their cache" What a waste of time!

Instead of alerting me "You reached your invoice limit" nothing was ever mentioned, and my client and I wasted hours on this issue. The support rep blamed me for not counting up the money i received and knowing that I was over the limit. I asked for a monthly credit for all my wasted time, a mere $20 credit if that, and they ignore me. What a horrible company.