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"Curse" That's where you lost my interest in posting seriously. Ever heard of "high numbers"? If you're in a stadium of fifty thousand people, chances are that they'll be a good deal of people with the same first name as you. It's like looking at a splattered pattern of paint, it's inherently meaningless, but people try to see images inside it when there's really NOTHING. There is no pattern in randomization, even if there appears to be.
While I'd agree it's probably chance, the odds of what happened happening aren't all that high.
"Indeed, all American presidents elected in a year divisible by 20 between 1840 and 1960 died in office:"
It also said, though I'm too lazy to look for it, that only two other presidents died in office, I believe.
I think it's more likely that, rather than just events matching some curse that was supposedly made, someone noticed that presidents elected in 20's have usualy died and made this up.