Well, first I should probably ask if this is browser dependent.

I've read that if an invalid token is found, but the section of code is valid until that invalid token, a semicolon is inserted before the token if it is preceded by a line break.
However, the common example cited for bugs caused by semicolon insertion is:

return
  _a+b;

which doesn't seem to follow this rule, since _a would be a valid token. On the other hand, breaking up call chains works as expected:

$('#myButton')
  .click(function(){alert("Hello!")});

Does anyone have a more in-depth description of the rules?