Garden path sentences are those that lead a reader down a path to an incorrect semantic and/or syntactic interpretation of the sentence during the initial analysis or parsing of the sentence. The field of psycholinguistics has shown that these incorrect interpretations occur because readers process sentences one word at a time.A simple yet cogent example of a garden path sentence is:
The old man the boat.
When reading this sentence the most likely initial interpretation (illustrated by the syntactic tree below) is that "old" is an adjective describing "man". But what happens when you get to "the boat"? This can't be a sentence without a verb.
By backtracking and reading the sentence again the proper interpretation with "old" being a collective noun and "man" being a verb can be reached. In other words, "the boat is manned by the old (people)." 
Similar to garden path sentences are newspaper headlines that are written in such a way as to lead a reader to an incorrect interpretation (often by omission of words). These headlines have been dubbed "crash blossoms" based on the following headline:
Here is a diagram of the likely initial interpretation followed by a diagram of the intended interpretation (with words added for clarification).
Parse 1:
Parse 2:
Language Log posted more about the evolution of the name "crash blossoms" here.And here are two additional crash blossoms that I find particularly amusing followed by their respective conflicting syntactic trees.
Parse 1:
Parse 2:
Parse 1:
Parse 2:
