If you hand-knit a sweater for someone you are dating, you are doomed to a sudden break-up.
The legend of the sweater curse has gotten a lot of blog press over the years and there are many first hand accounts of the phenomenon, online. A certain Ms. Dean actually uses it as a relationship test (read here). It has its own very thorough and serious Wikipedia article. I advise you to read that article in its entirety so that you are fully warned and prepared, but here are some highlights:
"The significant other may simply not want to wear anything hand-knit. A hand-knit sweater can also subject them to ridicule, either because the sweater looks bad (i.e., poorly made or unfashionable) or conveys overly domestic connotations."
Also, "a sweater may not suit the intended recipient's clothing style or it could make them uncomfortably hot."
"No controlled studies have investigated this phenomenon, and the available evidence is largely anecdotal , which is generally of low reliability due to cognitive bias."