I learned a long time ago that one of the hardest Tongue Twisters in the English language is "toy boat". If you're a native speaker and you don't trust me, try it - say "toy boat" quickly and repeatedly. If you're not a native speaker, go find one and get them to say it - you'll be pleasantly amused by their difficulty.
Other examples of this phenomenon in English are "big whip" and "unique New York", although this latter is far too contrived for my liking - the beauty of "toy boat" is that it's a phrase that you've undoubtedly said at some point in the past without realising it's potential for short-circuiting your voicebox.
Since I discovered this weird phenomenon - not just that "toy boat" makes for a ridiculously good tongue twister, but also that non-native speakers never seem to have any difficulty with it, I've been on the hunt for something similar in other languages. So far, I've found "piano panier". If you're a native French speaker, try it. If you're not (but speak good enough French to give it a go), just see how easy it is. I've yet to get any examples in other languages, but that's probably at least in part because I haven't asked enough people.
I'm also very intrigued as to why there is such a difference. Do our brains actually process speech in foreign languages fundamentally differently? Is it just that I pronounce "piano panier" so badly that I don't come across the same difficulty as a native speaker would? Are there any serious linguists out there who would be interested in studying this phenomenon?