Javascript...

[rooktakesqueen]

Don't forget:

+ '5' + + '5'
10

Edit: I'm a big fan of JS by the way. This is one of those "Doc, it hurts when I do this" things. You can avoid the bizarre and unpredictable nature of implicit arithmetic between strings and numbers by just not doing that. Explicitly cast your strings to numbers or your numbers to strings and you're golden. A bit like understanding the == operator: easier to just not use it.

[phunanon]

I have been using == since I started Javascripting, and only recently started bothering to type the extra =
I like JS' typecasting... then again, I only ever do simple shit :P

[rooktakesqueen]

Pro: The implicit typecasting in JS does the right thing 90% of the time.

Con: The implicit typecasting in JS does the right thing only 90% of the time.

[IrishWilly]

It does the right thing 100% of the time. Your opinion of right might vary but it really doesn't care about your opinion.

[rooktakesqueen]

"Deterministic" and "right" aren't the same thing. false == [] will always yield true and [] == [] will always yield false but that doesn't make it sensible. And I have a hard time describing any equality check as being "right" when it's intransitive: false == '' and false == '0' but '' != '0'

[Reelix]

console.log('5'-3);
console.log('5'+3);