2spare has a list of the top 87 bad predictions about the future, serving as a sort of catalog for the short-sighted. Some favorites:

"It will be years - not in my time - before a woman will become Prime Minister."
Margaret Thatcher, future Prime Minister, October 26th, 1969.

"Man will not fly for 50 years."
Wilbur Wright, American aviation pioneer, to brother Orville, after a disappointing flying experiment, 1901 (their first successful flight was in 1903).

"The horse is here to stay but the automobile is only a novelty, a fad."
The president of the Michigan Savings Bank advising Henry Ford's lawyer not to invest in the Ford Motor Co., 1903.

"There is no reason anyone would want a computer in their home."
Ken Olson, president, chairman and founder of Digital Equipment Corp. (DEC), maker of big business mainframe computers, arguing against the PC in 1977.

"The wireless music box has no imaginable commercial value. Who would pay for a message sent to no one in particular?"
Associates of David Sarnoff responding to the latter's call for investment in the radio in 1921.

(On the subject of rockets.) "We stand on the threshold of rocket mail." -"“ U.S. postmaster general Arthur Summerfield, in 1959.

There are tons more in the source article, and even a Wikipedia page on famous predictions. This all leads me to think...I've made some pretty bad predictions. The one I remember best is when, as a loyal CompuServe customer, I first saw the World Wide Web via an early version of Mosaic, in my brother's college dorm room. "What's this good for?" I asked. "Here, look at my home page," he said, bringing up a page of information about himself and his studies. I shook my head. "This web thing is stupid. Nobody's going to pay for this -- I'll stick with CompuServe." Sigh.

Have you made a bad prediction? Come on, I'm sure you have. Share in the comments, if you dare.