Why does the heart symbol sometimes turn into a question mark?

Question

On my myspace page I added some hearts using the alt and 3 and the symbol didn't stay, when I logged back on the heart was a question mark.

Answer 1

Alt 3 on PCs generate character 3, which is rendered by PCs as a heart. In fact, it's a control code and won't be rendered properly in most applications such as web browsers - since control codes (and all character below 32 are control codes) are not intended to be printed, they are generally replaced by a question mark in the more intelligently designed applications.
People use emoticons for this reason - they use normal character that render properly in most applications, but express emotions in some way.

Answer 2

Did you use a different browser?

That's usually the reason.

Not all the symbols are supported by all the browsers the same way.

OR, the PREVIEW you saw was interpreted differently that it was when later viewed through the browser.

Alt 3 sounds way wrong anyway