I've seen Thais act peeved (or as peeved as Thais are apt to act given that they rarely telegraph emotions
) when they're spoken to by a foreigner in overly formal-ish Thai. It's almost as if they think the foreigner is "takin' the piss", because that Thai is never addressed in that fashion by anyone else. Thankfully, for the most part, the Thais are pretty easy going, and ANY foreigner making an effort to speak Thai is usually given a "free pass" with any "fox-paws" (faux pas) they make.
Speaking too high above someone's station is nearly as bad as me speaking to everyone with แก-เธอ and using เรา insteada ผม when I hafta use a first person pronoun. Still at least I'm not a กู'er or a มึง'er
.. That just drives me up the wall when I hear foreigners spit those out when they're talkin' to Thais they don't know really well 
. Usually that's enough of a joke to take the "crankiness" out of them and deal with me..The only time I'll use กู is when I ask a Thai who's cranky with me, if they know who my father is รู้ว่าพ่อกูเป็นใคร
(which BTW is what all the hi-so thai kids say when ever they run amuck 
). It never fails to get a great laugh outta the Thais because; face it my father isn't anyone special here, in fact he's not even here at all!! 
Sometimes it's a tough balancing act knowing what level of Thai to use with the Thai you're talkin' to. I think we as non-native speakers will never be able to seamlessly transition between the registers like Thais do.
I used to always tell Thais that I spoke Thai with a สำเนียงแปลก ๆ, but I had one Thai tell me last nite that it's better to say it more along the lines of ผูดไทยแปร่ง ๆ. .. Dunno which is better because I sure have an accent that's awfully "black-black". . .
Sorry for the off-topic..
