When a vendor asks you to implement their API on a client site and you spend half a day reading documentation they send you to realise the API is nothing like the documentation.

Contact vendor who replies with:
"Oh, did I send you the link to the v2 API docs? We are on v3 now."

I had the same problem... except the vendor was PayPal. Had to debug a lot to find out why some ACH payments arent getting through to find that crucial info for that is nowhere to be found. We ended up making a workaround. Still... really something I would never expect from Paypal...