I'm sure there are certain circumstances when it's possible to OD through smoking, but I can't imagine any. You'd have to work extremely hard at it I suspect... during which time you'd develop a tolerance making it even harder, I suppose... The chances of OD are all but non-existent from smoking but maybe if you really went for it and sucked up line after line back-to-back you could. I wouldn't bother experimenting along those routes though...
When I first tried it, after a single decent line (smokers often refer to the length of foil covered in a toke as a line) I'd be feeling every one of the heroin side/effects listed in any textbook on the subject - drowsy (but energised,) euhoric, etc. It pretty much does what it says on the tin, to be honest. Like being smacked over the head with a pink, fluffy baseball bat.