Usually, a question mark indicates that the speaker is not actually asking for forgiveness, but is indicating disbelief or offended sensibilities; the phrase is usually spoken with rising intonation. With an exclamation mark, it's usually asking forgiveness for something that has been done, or a polite way to attract attention, and is spoken with falling intonation.
"Well, I think you're a bare-faced liar!"
"Excuse me? Did you just call me a liar?"
"Hey! Get out of here!"
"Oh, excuse me! I didn't realise you were in there."