[imtringued]

The part where most people give up is useful error messages and parsing recovery. Parsing a correct document with a grammar is really easy, to the point that nobody should be under the illusion that it is difficult. The moment you add error recovery, the things that are obvious have n ways to go wrong and you have to come up with test cases for each of them.

[sieve]

Recovery is simple depending on your syntax (and whether your language supports exceptions). You pick a few places where you trap exceptions, add it to your log and continue. My current project is the first one that might becomes public. So it is the first time I have worked a little bit on usability. So multiple errors at a time, not one.