Can't remember where I read it, but someone put it best during the DAO fiasco last year when they wrote "Imagine Javascript but your bank account accessible through the Document Object Model."
for(var i=0; i<arr.length; ++i) {
Solidity is a "statically typed language" with "type inference". In most of these, you'd expect i to be typed as whatever the type of arr.length is, but Solidity does not care, it sees `var i = 0`, 0 fits into a uint8 so a uint8 i is, it'll get promoted during the comparison and if arr has more than 255 elements it'll overflow and the loop is infinite.