I once saw a paper about incrementalism in politics illustrated with a Rube Goldberg cartoon of a someone slipping his foot into a door, kicking a camel that then stuck its nose under a tent, which pushed a ball to start rolling down a slippery slope, into a river that carried it with the current over the falls and wore away a rock.
All of those metaphors but one are about how one thing leads to another. Going over the waterfall is a point of no return, as is Julius Caesar’s “The die is cast.” A more humorous one for how the consequences of our actions are now unavoidable and soon to catch up with us is, “a little pregnant.”
Another related metaphor is that, according to urban legend, if you drop a frog into hot water, it will jump out, but if you heat the water slowly, it won’t notice before it boils to death. James Fallows at The Atlantic has made it a pet cause of his to debunk this myth. In reality, frogs do hop out of dangerously hot water—unless their brains have been removed.
In the specific context where you’ve done or received an illicit favor, and are now being blackmailed for it, we might say that someone “owns you,” or (more crudely) “has you by the balls.” If someone you thought was your friend betrays you outright, that’s “stabbing you in the back” or “selling you out.” If it’s a relationship where they make you think they’ll return your favors, but they never do, that’s “stringing you along.” If they’re just taking advantage while outwardly maintaining their friendship, they’re “walking all over you.” perhaps “like a carpet,” and someone who gets walked all over is a “doormat.”