I saw this at Freetricity.com.

Problem:
An apostrophe appears where it should not.

Explanation:
“The reason’s why are threefold:” — with an apostrophe between the noun “reason” and the letter “s” — appeared on a Freetricity.com Web page about the E2D Windmaster and the three reasons that less than 3% of U.S. homes have a renewable energy-source product.

An apostrophe has very limited use in the formation of a plural noun. Here are the situations:

1. to form plurals of lower-case letters;
2. to form groups of years;
3. to form plurals of numbers;
4. to form plurals of symbols;
5. to form plurals of some short words.

However, most authorities recommend omission of the apostrophe for situations 2 through 4, given that there is no possibility of misreading when the apostrophe is omitted.

Given that the noun “reason” does not match any of these five situations, we have the solution.

Solution:
“The reasons why are threefold:”