The first cursed thing I learned about the SAP system I had to integrate was that to submit data, I had to send non-standards compliant SOAP messages. Literally construct an XML document in a DOM, then run various regexes over it in order to break it in ways that worked with their shitty XML parser.
It didn’t have transactions. You sent in data, one row would fail, but all the others would get inserted.
Every day, more annoying shitty errors that required emailing arsehole consultants.