At a very large bank here in Australia & NZ, all XML messages going through the main message bus had a trailing space character appended to the end, which broke XML validation on the receiving endpoint.

So the solution was for all endpoints to trim the very last character - not just if it was a space, but to chop off the last character. Apparently this had been the solution for years.

This worked really well until one day someone (probably a new grad) saw the character issue and figured they'd fix it.

A bank-wide P1 incident occurred because every single XML message was now unparsable due to the malformed closing '</xml ' tag. Every single application in the bank had to do an emergency update on its XML parser.