Apparently...

...they contain raffinose-type oligosaccharides which make you F**T cause flatulence and phytohemagglutinin which is toxic and can kill you.

So, you have to soak them overnight in cold water then boil rapidly for 30 minutes and then simmer for an hour before you can digest them safely.
 
Oh, hang on a minute, that’s kidney beans…

That was close ;-)