Not with webby stuff but an XML library
For years this had not been totally compliant, as it would accept XML with a blank line at the start (which apparently the spec says is a nono for some reason). Eventually they decided to clean it up, without bothering to tell anyone, and our code that had relied on the library exclusively suddenly failed to load any files.
It turned out, the devs hadn't noticed that their library was itself generating these blank lines on saving, and we had to do a hurried check for, and remove them when loading. I believe they eventually responded to the howls of protest.
P.S. We still make the check - just in case.