So I was readjusting my guitar today and when I went to tune the strings, I noticed that simply resting my hand on the head stock caused the strings to go roughly 1/4 step sharper. Is this normal and I've just never noticed it? I did adjust the truss rod a bit during all this but its the way it should be.
More information would make this clearer to answer. Is it a through neck, or bolted? Is it a solid guitar or semi-acoustic? Is it new or quite old? Has it a vibrato system?
On some guitars, it's quite easy to change the 'tuning' by 'bending' the neck. Others are a lot more sturdy, but it's still possible. The strings are always trying to pull the head of the guitar towards the bridge - they're under a lot of tension, so the neck has to withstand that. Hence the trussrod. By resting yur hand, or pushing slightly, you're assisting that tension, so the strings will drop slightly in pitch. Pull the other way, and you're creating more tension on the strings, which will slightly raise the pitch. Hopefully (you don't say) the pitch returns when you let go.
On a bolt-on neck - which often is actually a screw-on, there can be slight movement between the neck and the pocket it sits in. This is not good. Check the (usually) four screws for tightness. Should be very tight. If it's an old guitar, and a screw isn't biting, simple solution is to remove it, put a matchstick or toothpick in the hole, and re-screw.
On a guitar with a vib. system ('tremolo') the tension of the strings is balanced by a number of springs. When you push/pull the head of the guitar, that's effectively changing the equilibrium between them. Change of pitch is inevitable. It's possible to change by maybe a semitone, but more than that, and you'll need a new guitar!
Strings have elasticity, like rubber bands. They need to reach an equilibrium before they "settle" and stop adjusting.
- every time you turn your truss rod, the strings will need to be re-tuned regardless if the truss rod is now "the way it should be;" the string tension has changed
- every time you put on new strings, they will need to be tuned over and over again for a while (probably days); they're being stretched for the first time and will detune constantly
- if the temperature in the room (or outside) has changed then your strings will change their tune
- if "resting" your hand on the headstock means accidentally bumping a tuner, then yes this happens and could be the reason; depending on how your guitar is made, the neck joint could be loose enough to where applying pressure on the headstock (like your body weight) can push or pull the neck (and strings) in fact some players do this on purpose for effect
Basically, it's more likely that your hand on the headstock had nothing to do with this at all. Probably what happened is you made a change (truss rod, or something else) and tuned the strings and they adjusted requiring more tuning.
and great question!
In addition to the other excellent answers, I just want to point out that this is totally normal.
In fact, while a tremolo bar accomplishes a similar effect (1/4-whole step bending) by moving the bridge, where the bar is attached, another method of achieving similar tremolo can be accomplished by applying pressure to the head/neck to gently bend the wood. Typically you won't get as fast of a bending effect as a tremolo ("whammy") bar, or physically bending the strings instead, but it can provide a more thorough tremolo.
To do this, you would either use your fretting hand to flex the neck (towards your body for up in pitch, or away for lower) while resting your strumming hand on the body for counter-weight. Alternatively, I've seen it done while sitting, using your leg or arm to brace the body of the guitar and pressing directly on the headstock with your strumming hand.
It's an interesting technique, and doesn't invalidate anything in the other answers, but something to be aware of. I would suspect it happens more easily with softer woods in the neck, but haven't verified this personally. Just a guess.