I'm thinking here of "standards" like csv and mbox that are almost impossible to handle with 100% reliability if you don't control all the programs that are producing them. It can get even worse with some niche products. I used to work with a piece of legal software that defined its own text format, and had a nasty habit of exporting files that it couldn't import. There was a defined spec, but it was riddled with ambiguities.

I'm coming to think that, when it comes to text formats, it's LL(k) | GTFO.