Will someone help me identify the errors in these sentences?

In each one of the following sentences, there are 2 total errors from a possible 17:
sentence fragment / run-on sentence
lack of unity / slang
mixed construction / comma splice
faulty parallelism / redundant word
jargon / dangling modifier
misplaced modifier / long lead in
cliché / wordy prepositional phrase
outdated expression or word / exclusive language

In brackets, I will identify the errors which I believe to be true:

1. Having arrived punctually on time for the exam, a written excuse was not needed. (redundant word, comma splice)

2. However, his exam performance didn't cut the mustard, Stan had not studied at all. (outdated word or expression)

3. The reasons were because Stan worked late, hates exams, and is prone to lapses of concentration. (sentence fragment, faulty parallelism)

4. I guess I don't need to tell you that, for Stan, the exam sucked. (I'm completely stuck on this one)

Thanks for any help!