When does pos: rel + pos: absolute fail?

Hey guys,

was wanting to ask this a while ago.

so taking the standard example of this:

.parent {
position: relative;
width: 1000px;
height: 1000px;
}

.child {
position: absolute;
width: 500px;
height: 500px;
top: 50%;
left: 50%;
}

The values aren't important here, it's merely an example. I would obviously need to add css3 transform etc.

So this works fine most of the time, but I noticed sometimes it just screws everything up.

I guess it's when it becomes a bit more complicated with divs inside divs which are inside other divs etc.

So what situations does this break? Is it down to layout issues? For e.g maybe someone forgot a clearfix or something?

What are the rules for this to work? Does the parent need a height set?

A lot of the time I just hope this will work, and 90% of the time it does.

Sometimes though, with complicated layouts things just break and I don't know what to do then.

Any help much appreciated :)

Cheers!