For you grammer fanantics

I was having lunch at a restaurant the other day.
"Can I have a filtered coffee?"
The waitress hesitated.
"Umm.....I dunno. Let me check."
She came back happy. "yes. you can."
I was happy too.
"Can I have the hamburger?"
Same look. "Umm... I dunno. Let me check."
What is up with this place? Do they always run out of stuff? Why is she having to check if they have coffee and hamburgers?
Then it hit me.
I should have asked, "May I have..." not "Can I have..."