What is the most weird grammatically correct sentence that you know? Brace yourself because it may put you in a spin for a while. The following is a proper English sentence...
"Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo."
Here Buffalo is used with three different meanings.
a) Buffalo, New York the city
b) The animal buffalo, in plural - equivalent to buffaloes
c) To deceive
So what it means is Buffaloes in buffalo who are buffaloed by buffaloes in Buffalo,buffalo buffaloes in Buffalo.
It is a grammatically correct sentence used as an example of how homonyms and homophones can be used to create complicated constructs. It has been known to exist since 1972 when the sentence was used by William J. Rapaport, currently an associate professor at the University at Buffalo. It was posted to Linguist List by Rapaport in 1992. It was also featured in Steven Pinker's 1994 book The Language Instinct.
Sentences of this type, although not in such a refined form, have been known for a long time. A classical example is a proverb "Don't trouble trouble until trouble troubles you".
Visit Wikipedia for more information on this sentence, including the detailed grammatical construct and other similar examples.