I said it was on purpose, I didn't say it was... Hm... something is not right with this sentence. I came across this before, I just don't know what's the problem. Sometimes to me it feels perfectly natural to say something is while meaning it isn't. Just like now, it's raining outside but I don't mind if someone would say it is not raining even if I see it is with my own eyes. Strange indeed.
Sarcasm? Although, this isn't how I'd use it.
Sarcastic -- "It's raining? I can't 'wait' to walk across campus"
Literal -- "It's raining? Crap! I'm going to wreck my new clothes."
Is that kind of what you mean?