I was browsing the Thorn Tree language forum and there was a language mistakes thread. Needless to say my coworkers were looking at me strangely since every few seconds I would be trembling violently holding my laughter. So... thought I should share the joy.

portuguese / brazil
my last trip to brasil in 03-04 i made quite the mistake the last day. everyone in town got to know that i really wanted to buy a hammock (rede) but couldn't find them anywhere. then all of a sudden there was an influx of northern brazilians in the southern town i was in that came to sell their redes. the southern part of brazil is full of gauchos (cowboys). so after i bought my hammock (there were guys waiting for us at our apt when we returned home...that's how well known it was that we wanted one!!) i immediately went to the hardware store and asked for...

"um gaucho para pentear (os meus cabelos) no rede"
(a cowboy to comb my hair in the hammock)

instead of "um gancho para pendurar o rede: (a hook to hang the hammock)

they told me they were all out that day but to come back tomorrow!! and i swear when i was leaving town i saw a few cowboys (gauchos) look my way as i rolled out of there w/ my new hammock (rede)

french
In a car with three frenchmen, one of whom worked with me as a post doc here in the states, I finally heard something I understood, a discussion of the tobacco pipe resting in the console between the seats. So, I jumped, having had two semesters of french, "Ah, souvent la pipe?" small chuckle all around the car. I then, since I'd been understood and slightly amusing, I followed it up with, "Le tabac ou l'autre?" The car erupted in laughter! I'd intended to imfer that he might be smoking something more fun than tobacco, of course. Normally when I screwed up my french, Gwen would inquire, "Do you know what you just said?", but on this occaision he immediately exclaimed, "I did not teach him that!" Many of you will already know that "pipe" is french slang for "blow job".
souvent = often, le tabac = a small convenience store, l'autre = the other

spanish / mexico
i was in mexico after about half a semester of spanish in high school and i thought i was hot shit. we stayed in a hotel with a kitchenette, and we wanted to make breakfast the next day, so mi amiga y yo fuimos al supermercado.
"tiene huevos?" snicker snicker snicker "claro que sí. pero debes saber que..." and he proceeded to explain why i should ask "hay huevos?" instead. "oh. lo siento. ahora estoy embarazada" snicker snicker snicker
TWO classic spanish mistakes in one conversation man. they must've thought i was the dumbest gringa ever.
1) both 'tiene' and 'hay' means 'to have' but 'tiene' is used for things that is with you. so 'tiene huevos' means 'do you have "eggs"' instead of 'do you have eggs to sell'
2) lo siento = i'm sorry, ahora = now, estoy = i am
3) embarazada = to be pregnant, avergonzado = to be embarassed

arabic
Syrian Border guard: Where are you from?
Me : Aerlanda! (Ireland)
Taxi Driver and Border Guard: He is from Aerlanda Ha Ha Ha! (Aerlanda means dick-land, earlanda ,means Ireland)

spanish, portuguese / chile, brazil
In Brazil, there is a famous woman called Xuxa, who presents a TV Show for children. In this show, there are many young girls helping her, that are called paquitas.
In Chile, Xuxa (probably with a different spelling) is a slang for the vagina and paco is how the policemen are called by the population (like cop). Paquita would be a diminutive and female word for paco.

A friend of mine almost got into serious trouble when he asked an officer in Santiago if he had ever seen the Xuxa Show with the paquitas. It happened just at the end of the Pinochet time. Some years later, the Xuxa show was transmitted in Chile, making the children laugh and the parents cry.

spanish
I was in Cuba, first time trying my spanish outside of classroom. I met a friend who was injured after a basketball game. As I want to be helpfull, I asked if he had put "helado" (ice cream) on his anlke instead of "hielo" (ice)... He looked at me as if I were crazy and said "NO"!

chinese
4.00 am, China, Xiamen
A tired foreigner to his noisy neighbors: 'I want to sleep! '

Neighbors: 'Huh? Sorry we, don't have any dumplings!'

Turns out, the numbers stand for tones: shui3 jiao3 = dumplings shui4 jiao4 = sleep

german
A postcard I wrote to my German girlfriend's mother, whilst lying on a beach in Africa:
"Ich bin Heiss, Du Bist Kalt, Ich Bin Rot, Du bist Blau" which rather than meaning I'm hot, you're cold, I'm red, you're blue" translates colloquially as "I'm horny, You're frigid, I'm a Communist, You're a drunk."

english
When I was slingy drinks in a bar in Canada, we had a lady who was from Poland, who used to make some funny mistakes. Two large cocks please! (cokes, that is)