1. Finding out that said partner is quite different than the way he/she/it presented itself (mentally unstable, stalker, relationship addict, member of the opposite sex then previously thought, etc).
2. Mounting attraction to said partner is used to throw unwitting person to the less attractive or socially inept friend of out of your league partner.
"Hey Ted, what happened with you and that smoking hot girl at the club last night?"
"Oh yeah the next day she came to my house and had photoshopped pictures of us to look like our wedding day."
"Damn man, you got Shanghoed"
"Tell me about it."
Outcome 2 Example
"So what happened last night with that girl?
Seemed cool enough, then she introduced me to one of her sorority sister who dresses up for medieval fairs.
You got shanghoed my friend."