not as colourful as the original:

There is no accounting for human idiocy.

same here:

You can never imagine how stupid people can act.

den Spruch kenn ich im Hochdeutschen als "So dumm wie es manchmal kommt, kann man gar nicht denken." also so ähnlich.

I would have said "there's no limit to human stupidity/the stupidity of man" or "the sheer extent of human stupidity boggles the mind."

If you want more color, you can find some quotes on stupidity here: http://thinkexist.com/quotations/stupidity/

I tend toward this one, attributed to Einstein: "Two things are infinite: the universe and human stupidity; and I'm not sure about the the universe.” The page also attributes "there is no sin except stupidity" to Oscar Wilde.

But, of course, do some fact checking before you go and use any of them. :-)

All in all: Your Swiss grandpa was way ahead of his time! ;-)