Sentences that are incorrect but still understandable.
If you then include leet speak, acronyms, short form writing (SMS / Tweets), it quickly becomes unmanageable.
From what I understand, the modern understanding is that these point to the failure of grammar as a prescriptive exercise ("This is how thou shalt speak"). Human speech is too complex for simple grammar rules to fully capture its variety. Strict grammar and lexical rules were always fantasies of the grammar teacher anyway.
See, for example, the following article on double negatives and African American Vernacular English: https://daily.jstor.org/black-english-matters/.
For the record, the parser I worked on ended up having the "interesting" rules removed, leaving it as a tool for finding sentences that didn't conform to a Basic English grammar with a controlled vocabulary--and used to QC aircraft repair manuals, which need to be read by non-native English speakers.
I see it as a complete waste of my youth, BTW. Today I speak English that I learned through listening, reading and watching, and all of this mother tongue grammar nonsense that used to stress me out daily at school and during homework is absolutely useless to me.