Despite having spoken French all my life, I have trouble with the word “arbre” (tree). It’s the rbr combo that kills me. It just stays stuck in my throat and I end up gargling the damn word! So I try to use the specific tree name instead, whenever I can.
Yeah, and I don’t talk about Sherbrooke street much, either.
idea. Anytime I’m not careful, I always tend to add an “r” at the end.
. . . are you * Tim Russert*???
though I think that was Rodrigue, but same idea. . .
Texts and phenomenon.
hiatus
Luckily, on the rare occasions when I have to say it, someone jumps in and says it for me.
Marlboro
There aren’t any English ones that give me troubles, but damn, some in Spanish are just impossible.
Desgastado and desagradable, especially. There are others I can’t think of, but I know they’re out there.
I hate the word superfluous. I still can’t pronounce it correctly half the time.
I also have a great deal of trouble with exacerbate, which is unfortunate as I work in a hospital and it comes up a lot. Blech.
I have trouble with linoleum (naloleum), and tabernacle (tablnacker). I only need tabernacle when I am talking about the Mormon Tabernacle Choir, and for the other I just say floor covering.
I have trouble with the name “Royal York” (the subway station in Toronto where my video store is, among other things).
Royal York. Royal York.
Royal York.
I can feel the back of my tongue trying to tie itself in knots.
Welcome, Miakoda! Did you know that your username is perfect Esperanto? 
Entrepreneur. I want to pronounce the E like an E, not an O.
Ententantunje
It’s not a real word, naturally, but I have trouble saying it every time.
Edited
…as in Ed-did-a-did-did as I pronounced it in a Tech writing position interview. :o
During a college political science class, I was reading a passage from Thomas Hobbes’s Leviathan aloud. I was doing perfectly well until I came to the word “ceaseth”, which I stumbled over repeatedly until the professor told me to just say “ceases” and move on. Fortunately, the word “ceaseth” is archaic, and I haven’t had cause to say it since that day a quarter-century ago.
I have trouble with phrases that contain a rapid succession of several r and w sounds, which means I try to find alternatives to “one-run win” if talking baseball, or “white right-winger” when discussing politics.
“blood pressure”
Not a phrase I have to say often, but the /l/ and /r/ close together trip me up.
Me too. Comes out like /mallboro/. Damn those letters in proximity to one another!
For a scene in an acting class, I had a line that read “The soldiers’ll come!” I could not wrap my tongue around the middle word to save my life unless I went really super slow.
Sixths. The most unpronounceable consonant cluster in the English language. The word that forced me to give up a promising career as an elementary math teacher.
You think English is bad, try Georgian or Tibetan.
Vprtskni?
I think I need to add your screenname to my list of tongue twisters 
I’m a bit surprised no one’s mentioned “specific”, I thought that was a fairly common one. Knew a girl a while back who used to fumble it so badly and often that she found it easier to say “pacific” and just explain herself afterward when someone asked.
Vprtskvni – ‘I am peeling it’.
mkha’ 'gro bde chen rgyal mo - ‘Sky-faring Dakini Queen of Great Bliss’
Okay, so I left out the second V. Sheesh.
:: counts ::
Yep. Nine-letter word, one vowel, the vowel at the end… I was amazed when my friend told me about it. I suspect that at least the R and the first V are vocalic, no?