Ok, just been through this in a demo.
Parallax error. Caused by the thing you are measuring with not being in the same plane as the thing you are measuring... ie if you drive along at 70, your passenger might see 75 because the speedo needle is not sitting on the surface of the gauge but perhaps 5mm above it... so if you shift your head, the reading changes. It's why rulers are thin, so they measure what they lay on, not allowing for any parallax error with them not being close to what they are measuring.
In scopes, the image is pulled in from the front lens and focused at a point. Like cameras, scopes can only focus to a single point at a single range. You eyepiece is focused on the reticule, which ideally should be at this point. However, because scopes focus at a single point for a single range, this focus point may not be exactly in the same plane as the reticule... it may be a smidge forward or behind... not such a big deal, except you are looking at it at perhaps 10x or more, so the error by these two moving around when your head moves (in the same way as the needle on a speedo does), is magnified. What also is magnified by magnification, is that you might notice it's not so in focus. Which is why the FT mob use high mag, so they can see when the scope is perfectly in focus, and if you took the time to focus the reticule beforehand, when the two are perfectly in focus, you will have minimised the PA error to nothing, or very close.
Obviously the reverse is true, PA error is less noticable on low mage scopes, where the apparent
movement is reduced and the focus looks good. However the PA error is still present if it's not dialled out.
Assuming you have focused the eyepiece to the reticule first, sharp focus is a by product of low parallax error, but if you have a scope with a wide focus range (ie a small objective, low mag scope) be aware that your focus may look sharp for a wide range of distances and consequently you may still have PA error. But really, unless you have a high mag scope, one good way to test is to move your head around... if the crosshairs move across the target, it's not PA'd to that range.
I cant remember which is worse, a far focused scope being used at close range, or a close focused scope being used further out... you might want to play with that a but.
Also, if you have a scope that can resolve detail greater than your eye can, you may not get on well with it. Having astigmatisms, my eye goes in and out of focus, so i prefer scopes with definite focus, rather than something like a leup which can resolve really good detail that my eye won't hold on because it's focus wanders. Past a certain point, there is no benefit to me in seeing the nth detail, because my eye won't hold it.
I think i need to write this up with some pics... but work is mad at the moment, so don't hold your breath