I have a problem with the track pad on my Mac Book Pro (mid 2012) The trackpad on my mac book pro (mid 2012), isn't functioning properly. Sometimes it behaves as though I'm holding down a click. For example when working with a text editor I move the mouse across the page and the editor selects the text I move across (without me clicking). 

Sometimes it's not possible to click even right at the bottom of the track pad - it feels the same when you try to click right a the top of the track pad (i.e. there is no movement and no 'click' sound).

I've got Apple Care, but the error comes and goes so when I took it into the local repair centre they said they couldn't find anything wrong with it. (It's an apple authorised repairs rather than an actual apple store).

Any suggestions what I can do? What might be causing this? How I can reproduce the fault for the repair guys?
It sounds like your track-pad might need to be adjusted. If it's too tight it can cause the "symptoms" you are describing. I would say this is most likely, but it's always possible that there is another issue with it. If this happens to be the problem it is an easy fix, really. Or, at least it should be. I would take it to your authorized Apple Care repair shop and just have them open it up really quick and take a look. 

You can do it yourself without much difficulty at all. Just keep in mind that if you open it up yourself that it voids your warranty. If you are OK with that, then go ahead and check out the solution below.

You need three screwdrivers to get in there and fix it: a tri wing screwdriver(same as the one for nintendo), a Phillips #00 screwdriver, and a T6 Torx Screwdriver. The screws holding the back cover on are tiny phillips screws, the battery is held in by tri wing screws, and to adjust the track pad you will need the T6 Torx driver. NintendoScrewdriver.com has a nice little screwdriver set that has a precision tri wing and phillips screwdriver in it (find that here: nintendoscrewdriver.com). It's technically for Nintendo stuff, but it's high quality and you can't go wrong with it. You can find the T6 torx driver here (also good quality, so you don't strip screws and so it actually fits!). 

You really just need to take out the battery to get to the track pad. This video does an awesome job showing very clearly how to do this: (www.youtube.com/watch?v=aQZ_rG9VpHk). The parts you need are the first 0:00-1:45 (shows how to take out the cover and battery) and 13:40 shows taking out the track pad. You won't need to take the track pad out unless you are going to replace it. Also, this video (www.youtube.com/watch?v=BdKwE9FOnrI)does an AWESOME job describing how to adjust the track-pad. The macbook in that video is a 2008, but it should be the same on the 2012 model. I would also try cleaning it out with compressed air after opening it up.

Another possibility is that the battery is swelling. This is actually somewhat common. You can read about that here: discussions.apple.com/thread/4652016

If worst comes to worst, replacing the track pad isn't too bad. But, rounding back to the beginning, try to use that AppleCare to resolve the issue first so you don't void the warranty.

Hope this helps!

Note: you will have to copy and paste some of the links. This is a new profile and it won't let me post more than 2 links until I have over 10 reputation points.