Carsten Becker wrote:

> You mean unaspirated [p t k], don't you? [R] isn't that hard
> for me. Neither is [X]. ;) I don't know how to pronounce [q]
> properly, though. I always haven an uvular offglide there
> somehow.

Yes, there are certainly some contexts in which sounds like [q] are hard 
to pronounce correctly, [qi] for instance. (In Olaetian, which as my 
first language turned out to be the one that many of the interesting 
"foreign" sounds ended up in, /q/ can only occur before back vowels; a 
sort of [M]-like glide written as "u" intervenes between /q/ and a front 

But it was really just [k] (unaspirated) that was the difficult one for 
me; [p] and [t] came much easier for some reason I don't quite 
understand. I probably learned them subconsciously while learning 
Spanish, but that doesn't explain why I didn't get [k] right.

The only time I was able to reliably pronounce [R] was when I was 
learning German and actively practicing the sound. But I guess I've 
exaggerated its difficulty a bit. I was thinking mainly of sounds 
represented by IPA characters, and didn't consider all the variations of 
sounds that require diacritics (not to mention the ones not easily 
represented in IPA at all, like those Korean stops Henrik Theiling 
mentioned). I'm guessing the weird American English "r" is probably as 
difficult for native German speakers, even though it's a relatively easy 
sound for native American speakers like me.

  But yeah, there are many sound I don't know how to
> pronounce, especially most of the central vowels except [3],
> [@] and [6]. I attestedly don't get [1] right as well. A
> Russian friend of mine: "Ah, it's typical. Germans *never*
> get that one right." Better don't ask me about [_j] either.
> However, it also seems typical for German speakers to have
> difficulties with [H], i.e., many people in my French class
> say [pyi] for [pH] <puis>. Also, vi Tshermens are known for
> mixing up [T] and [s] and [w] and [v].
> Carsten

I haven't got a good handle on the central vowels either, and it doesn't 
help that no two IPA sound sites agree on what they're supposed to sound 
like. But not many languages have vowels exactly aligned with the IPA 
symbols anyway. It's easier to just listen to a recording and imitate 
than to try to make sense out of the often inconsistent IPA 
representations of vowel sounds (which I'm guessing may be due in some 
cases to historical changes in the set of sounds that had IPA symbols).