Advanced sniping ... I have a difficulty

I'm a decent sniper.  I'm not as quick or as accurate as some, and definitely need a new mouse (I'm using a wireless, which is nowhere near as accurate as the mouse I'm getting).  However, I've found one difficulty in sniping (and, conversely, in avoiding getting sniped) because... I think... of the way the Source engine communicates a player's position.

Has anyone else noticed this?  You'll have a headshot lined up, pull the trigger, but it doesn't register as a headshot.  Or you poke your head out for a split second, see the sniper, and then get back behind cover, only to die, when you obviously are out of the line of sight of the enemy.

Here's an overview about how the online prediction and modelling work... 

http://www.valve-erc.com/srcsdk/general/multiplayer_networking.html

it's obvious to me that some people have mastered leading the player or reacting in a way which takes this into account, because I see people getting one-shot kills with a lot more consistency than I do.  Does anyone have any tips about nailing someone, improving accuracy with this in mind?  Where do you aim to get the headshots, because pulling the trigger with the dot on their face, at least from my experience, isn't foolproof, and I think this is why...

I've noticed both of those issues myself.  I'm a fairly proficient sniper, not great but not terrible.  I've had people in my sights, top portion of the head, pull the trigger and no headshot.  Gets somewhat annoying, especially when it happens a few times in a row.  I usually just try to hit the topmost portion of the head and that helps it out a decent bit.  As far as the other issue, the only thing i've noticed that helps is to aim a very small bit ahead of the head and you can still get headshots that way. 

I have found that on laggy or crappy servers i pull off a clean headshot only to get sniped back, from the one i shot, and when it zooms in, there is blood all over the opposing sniper's head but no damage was done.  I think that is mostly a lag issue, i am interested in honing my sniping skills so i hope we can get some insight from talented snipers on this.     

This happens to me all the time when I try to snipe a heavy.  Get it right on him and it's almost always a critical hit, but I don't see him fall.  Normally their's a medic on him, and I thought that might affect it somehow, but it did the same thing on a demoman once.  It makes me sad =\

Heavy w/ medic has to be fully charged headshot, anything less wont quite do it.

AS for the original posts question, I've noticed a bit, but to be honest more times than not if I aim for the head and pull the trigger it works out. Its possible that some of the time (not all the time, cuz as you pointed out there is occasionally some kind of delay) you  may not be charged enough in your shot to take out the class you're shooting. Maybe they just got a medic buff you didnt see or something.

Anything else is probably either a miss, lag, or both.

I'm not totally sure this isn't an accuracy thing put in place for balance.

Just a thought.

I have seen/made crit one-shot kills that appeared to me to completely miss the target.  I attribute it to lag.  That sort of thing happens a lot where two opposing actions happen nearly simultaneously on each users client.

I tend to play offensively on TF2 and so I've quite naturally been sniped alot during my career. Lately I've been taking up the sniper reigns and been having more fun with this class. My beef lies with the fact that when I get shot by a sniper about a quarter of the time I've ducked behind a corner or an object and I still get sniped. In many cases Im a good 5-7 feet behind the object and I still get sniped. Lag or connection speed doesn't seem to matter either. Several times I have been over 10 feet behind a wall as I duck away from his shot and I still get headshotted. Lamo! As a sniper I have yet to do this. Grrrr! very unrealistic in my opinion.

It's realistic when your head is poking out and even though u think i can't see you, i can.

Froth, it is not necessarily your lag that would be doing this, but the other snipers.  If either player is slowing down then this will happen quite often.  Although it seems to always effect both snipers in the same way.