The user sets out to do one thing, but a different, undesirable thing happens instead.
This is one of the oldest tricks in the book, and it is very broad in nature – many dark patterns involve some kind of bait & switch.
Clicking the cross in the top-right hand corner of the pop-up box now agrees to a scheduled upgrade rather than rejecting it.
A Roach Motel makes it very easy for a user to get into a certain situation, but then makes it hard for them to get out of it when they realize it is undesirable.
The user attempts to purchase a specific item.
However, somewhere in the purchasing journey the site sneaks an additional item into their basket, often through the use of an opt-out radio button or checkbox on a prior page.
The user is required to respond to a question (typically in the checkout process), which, when glanced upon quickly appears to ask one thing, but if read carefully, asks another thing entirely.