This is precisely the sort of thing which crosses my mind when talking about the possibility of giving offence in a second language. If someone whose first language is not English comes out at random with something offensive I would tend to assume that they had made an error - obviously if they are upset with me and call me a camel's arse or whatever it is likely that they meant it but if we were otherwise exchanging pleasant small talk I would assume first that they had mishandled an idiom or that perhaps a completely or partially wrong word had popped into their head - which is not uncommon.
There are probably some French who just don't like the English and will take offence easily but I would hope that the average person would realise that speaking a second language is rarely perfect.
Tu vs Vous I find easy enough but only because I know no French speaking adult well enough at the moment for tu to ever enter the picture and I don't generally speak to any French children.
But I do nearly always say "Je peux avoir.....?" in shops because I pretty much always say "Can I have....?" in English in the same situation. I know in theory that "Je voudrais" (or Je pourrais, but "Je pourrais avoir...?" is too much of a mouthful) is more polite but rarely remember to use it.
Am I inadvertently giving offence or will the average, and sensible, Frenchman smile quietly to himself and merely think "Ils sont fous, les Anglais"?