This issue spreads throughtout the entire day so bear with me please.
DP and I had been very busy all day and hadn't had time to eat. We were driving past a McDonalds and I asked him if he fancied grabbing a burger. He agreed. We go through the drive-thru and I ask for a big mac meal and DP says he wants a chicken sandwich meal which costs £2.19. I order both and DP says "I'll sort out what I owe you when we get back to your house". I was like " 
don't worry about it, I'm buying". He didn't respond which I thought was rather rude but anyway!!
Later in the day we're in Asda and I have a conveyor belt of stuff heading to the cashier. DP puts a few things on the end of it, hands me a £5 note and says "that's for the stuff, you owe me 50p". 
I laugh assuming he's joking, turns out he isn't - I look at the stuff he's got and it can't cost more than £3 so I ask "why 50p?? that stuff doesn't come to £4.50" and he says "no, its to cover my side of the Mcdonalds earlier." At this point I see the couple in front of us catch each other's eyes and smirk without saying a word. It's obvious they were laughing at us.
I said "I bought the lunch, I told you at the time" so he says "oh did you?? oh right!! cool, well you owe me more than 50p then! () here let me work this out ... " the cashier then smirks and the man in front shakes his head. All 3 of them are ripping the piss out of us. I snap "Just sort it out later for gods sake".
We had a row about it when we got back to the car. I feel he showed us up and he does this a lot. When we got out for "2 meals for £10" things he'll try and work out exactly how much I need to contribute in front of the waitress. He's done it on buses, pubs, everything. One time we were in a bar and I'd bought the last round. We went to the bar and looked expectantly at him and he said quite aggressively "oh no! it's YOUR round!". que - a group of blokes start laughing at him. I calmly point out "err no, it isn't" to which he replies "Of course it is!!" when he realises his mistake he points out that as I drank a coke on my last round I should make up the difference or some shite that I didn't fully understand at the time.
Anyway, past issues aside, was I being unreasonable to be more annoyed at him making a laughing stock of us in asda than be annoyed at the people laughing?