Very often the code needs to check a lot of things to make sure the final operation it needs to perform will not fail. First the classic, positive construction of these conditions, then the bail-out version:

/* classic */
function do_something()
{
	prepare1;
	if (valid1) {
		prepare2;
		if (valid2) {
			prepare3;
			if (valid3)
				doit;
			else
				error3;
		}
		else
			error 2;
	}
	else error1;
}

/* bail out */
function do_something()
{
	prepare1;
	if (!valid1) {
		error1;
		return;
	}

	prepare2;
	if (!valid2) {
		error2;
		return;
	}

	prepare3;
	if (!valid3) {
		error3;
		return;
	}

	doit;
}

Pros for the bail-out structure:

• the validity check and the error handling stay together
• prefer the simple, flat logic where possible
• it is very easy to add or remove new checks, without having to introduce new levels
• even temporarily disabling a check, e.g. commenting it out, is much simpler
• as the code grows, functions often need to be split; the flat organization makes it simpler to split out a set of checks in a separate checker function (see below)

function layer_checks()
{
	prepare2;
	if (!valid2) {
		error2;
		return bad;
	}

	prepare3;
	if (!valid3) {
		error3;
		return bad;
	}
	return good;
}

function do_something()
{
	prepare1;
	if (!valid1) {
		error1;
		return;
	}

	if (!layer_checks())
		return;

	doit;
}