Seriously, if you really want to rattle a Thai person in Bangkok, then speak to them in their own language. Truly, there are few things that irritate them more than a foreigner who can speak Thai clearly and is, or appears to be fluent. It is akin to waving a red flag at a bull.
To illustrate, I am referring to the kind of Bangkokian that stands beside you and repeats everything you say to a third person, that is in Thai whilst you have just said the exact same, in Thai! Could there be anything more nonsensical than this! Well, perhaps another ubiquitous situation where a foreigner speaks in Thai to a Thai person, while they insist on speaking in English, however broken or incompletely.
Let's have a quick think about that one. Two people conversing in two languages in one conversation, not only but also each speaking the other's first language. Mmm, does this form of insanity happen outside of Thailand I have often wondered to myself?
The third most regular scenario, is when a Thai person who can hardly speak a word of English, has no other option in order to retaliate than to pretend that he/she doesn't understand what you're saying even though they have full comprehension. Occasionally, he/she having stood for a period of time with a confused look upon his/her face, will then run off and return with someone else, often a person with similar linguistic limitations. And next you are subjected to 'What!' or 'Yoouu!' instead of alternatively 'kortort thi tae phud thiy di him thi' or (Excuse me, but can you speak Thai?)
More significantly is the actual reality that in the 39 years that I lived in the UK or/and travelled to other countries, I never tried any of the above tactics to piss foreigners off. In other words, I never stooped to any of these to save my face, just because I wasn't proficient in the first language of the person who was speaking to me in my mother tongue.
In all the years I've been here, I've never had a single Bangkok native who has asked me sincerely, if I can speak Thai. Whereas, I do get it now and again in the rhetorical, for instance having just replied that I can in fact speak Thai. Then the questioner will not react in the slightest, apart from by entirely ignoring my answer.
Another is when somebody asks to hear me speak Thai and then swiftly holds up their hand and spouts 'Enough!' Or adversely, having given it some thought, the enquirer will go into one of the longest Thai sentence structures I have ever heard, whilst mumbling thirteen-to-the-dozen. And when of course I shake my head and tell them that I didn't understand, they shout something along the lines of, 'Ahh!' scowl, and point their finger at me to convey that they have proven me a liar.
A word of advice. Don't look flustered or frustrated because they thrive on that, it is all the encouragement they need to continue on their theme and raise your blood pressure through the roof! Just relax, smile, ask them if they can speak Bangkok Thai, or is it that they are in fact from the Northeast of Thailand. Being mistaken for a native of Issarn (my good wife's area) sometimes nudges a Bangkok native to speak colloquial Thai. As they feel they are superior to their rural cousins and feel the need to prove themselves of different stock.
Communicating in other regions
Yet, why do I seek to tarnish the Bangkokians and not those Thai people of other regions. It is because they have not rallied to drive me up the wall and down the other side. At least, when I have gotten away from the provinces that surround Bangkok, higher upcountry or deeper down south.
For example, when I have spoken to Northeastern Thais in Ubon Ratchathani, and have used the incorrect quantifier as Issarn or Loatian can be distinctively different from Bangkok Thai. Far from behaving ignorantly, they have generally explained what it is, that I should have said in order to be correct. In plain English, I have always assumed that this is so I should know the next time, and that they are proud of their distinctions.
Further example, when I used to speak to Southern Thais in Hua Hin, on a regular basis in their own language. Their reaction was usually to smile or even laugh in bemusement, to hear a foreigner speaking Thai. As well, they would sometimes wave over a friend of theirs from another shop or market stall, to allow them to listen to the novelty.
My guess is that far fewer expats per capita can speak Thai there, as Hua Hin is partly a retirement town for foreigners, and older Westerners find tonal languages too difficult or cannot be bothered. And obviously down to the reason that the Thais there are geared up to cater for foreigners, therefore there isn't the need to learn Thai. The locals there are able to speak other languages.
Most importantly, being spoken to in your own language, probably is not all that offensive, if at all, when you are confident in your own ability at speaking another person's language.