What can happen if a recommendation letter falsifies information?
Few cases can happen (probability will decrease as we go from case to case).


(They don't find out) nothing happens.
(They find out) they'll remember the professor and his recommendations will be considered of no value 


The reader of this recommendation can have the professor marked as a dishonest person within the facility or keep it for himself

They contact the professor ask him about it. Then they contact the current professor's employer and he gets in 'trouble' - some people get aware of it.
They contact current professors employer, he get's fired for dishonest behavior
They make a lot of people aware (Including his colleagues and newspapers/media?) he gets fire and it's going to be hard for him to find another job, highly unlikely.


It really comes down to the credibility, you're not going to jail because of that. It can only really hurt your trustworthiness in the academic and human sense. If you're going to lie it should be something small like if someone worked in your company for a year you can say he worked there one and a half years. 

If the one recommending someone isn't aware of something chances of this affecting him are very low ~0. This is in the case of something that common sense dictates, he shouldn't be aware of I don't know, Like does he have a dog, and on the other side, one can't say he's was not aware of his e.g. effort or performance.