I've been using the French course daily for around a week now, and I think I've seen some grammatical mistakes in French sentences:
Nouns without determiners
Nouns often show up alone, especially after faire or avoir. I tried to look for an explanation, but I haven't found any source stating that objects of faire or avoir don't need any determiners. I've seen these very often.
Des followed by an adjective
When followed by an adjective, the plural indefinite/partitive article should revert to de, but I've seen one or two sentences where it remained as des.
- Noun adjuncts
Sometimes noun phrases are composed solely of nouns. I think this is unacceptable in French, where you have to use de when combining two nouns.
However, since I'm still relatively new to French, I could be wrong - please notify me either way!