Not my story, but amusing anyway. I found it hysterical, but I'm a jerk like that.
My friend worked for a bilingual company this summer (I'm in Canada). All employees were required to greet all patrons in both official languages and then use whichever one the person responded with. A lot of people, especially since we're so close to Quebec, are VERY touchy about which language you use with them.
Here's how the conversation was supposed to work:
Friend: Hello Bonjour
Patron: Hello!
Friend: *speaks English*
OR
Friend: Hello Bonjour
Patron: Bonjour!
Friend: *speaks French*
Here is what USUALLY happened:
Friend: Hello Bonjour
Patron: Hello Bonjour! :D
Friend: ...*starts speaking in English*
Patron: OMFG WTF I SPEAK FRENCH YOU HORRIBLE PERSON AHARHGSFS DIRTY COLONISTS
OR
Friend: Hello Bonjour
Patron: Hello Bonjour! :D
Friend: ...*starts speaking in French*
Patron: ....I don't actually speak French. THEY ARE MORE ENGLISH ABLAHBLAH CULTURAL ASSIMILATION
OR
Friend: Hello Bonjour!
Patron: Hello Bonjour!
Friend: *speaks English*
Patron: .....?
Friend: *speaks French*
Patron: ......*speaks Chinese* :D
Friend: Awww :(
Patron: Y U NO SPEAK CHINESE?!
I always greet her with 'Hello Bonjour' now
- Allo-Bonjour!

But you have to say it 'AlloBonjour?' to have the full effect
Then of course they'd launch into French I'd feel so bad about my unbelievably sketchy French that I'd avoid talking to retail people at any cost.
"Excusez-moi, ou se trouve la toilette, s'il vous plait?" (Anglo accent very much in evidence despite French immersion since kindergarten.)
"It's over there."
*thinking, "Okay, we'll speak English, then"* "Thanks."
"Pas de probleme."
That could definitely be confusing and/or irritating, though.
And I will totally go around muttering "Hello Bonjour" all day today.
If I do it on the phone at work, I'm so blaming the OP. :P
...I was going to order in French too. :(
"So long, farewell, auf wiedersein, adieu..."
Me: Hello bonjour
Them: uhhhhhh...
Me: Hello bonjour?
Them: WHY ARE YOU DOING THAT I DON'T UNDERSTAND FRENCH CAN'T YOU SEE WE'RE AMERICAN STOP SPEAKING FRENCH TO US
Me: .....sorry?
It was great fun :) (of course not all Americans were like that)
and if something like that ever happened to me...with out any thought the words would slip out "gomen nasi"
I am far too used to chatting nonchalantly with my sister in a combination of English, German, Spanish and Japanese. Granted i only know catch phrases in German, Spanish and Japanese, i just use those phrases in liu of English far too much in my own home that going out into public it comes out with out thought.
I've been known to spit it out when appoligizing to customers XD Definatly gets me some odd looks :P
Its also fun to confuse people when out shopping...:P "Meine Schwester! Matte!" German and Japanese there. I dont call my sister by her real name mostly. Meine Schwester became her pet name.
/end rambling
I have a friend who is a Montreal citizen...
I find myself wanting to do it sometimes.