After reading a grammatical discussion on Sugarquill, I felt like making a
list of all the grammar errors that I find understandable/forgivable in
amateur writers. Of course, being the proofreader from hell, I would still
correct these errors, but I can understand why someone might make them, and
sympathize completely.

1. Not using the subjunctive in English. Misusing it is still wrong, but
not knowing to use it at all is understandable.
2. Applying English pluralization to words that take Latin pluralization.
However, the reverse is not acceptable.
3. Minor errors in reported speech tense sequence when the sentence
requires three or more tenses.
4. Faulty noun genders (fiancé vs. fiancée), ONLY if the writer has not
studied the language from which the gendering arises.
5. Singular "they". I don't even consider this an error - I want to
standardize it as soon as someone lets me edit the style guide!
6. Sentences ending with prepositions, as long as the preposition is not
one that should go with "whom."
7. Using "who" instead of "whom" when there is no preposition, and
confusing "who" and "whom" in convoluted sentences.