I was a startup where someone got an unconditional signing bonus. It wasn't deliberate, they just kept it simple because it was a startup and they thought they trusted the guy because he was an old friend of the CEO.

The guy immediately took leave to get some medical procedure done with a recovery time, then when he returned he quit for another job. He barely worked, collected a big signing bonus, used the company's insurance plan for a very expensive procedure, and then disappeared.

From that point forward, signing bonuses had the standard conditions attached.