To me it sounds a bit like what happens on my ap1 tranny in first at a stop. Basically you push the clutch in and put it in first, and it feels like it goes in gear but in actuality it isn't all the way in. If I take my hand off the shifter and start to move it will pop back out, if I give a very gentle pressure as I let the clutch out the gears will move the fraction of a rotation needed for the gears to line up the rest of the way and slip effortlessly into gear.

I've seen this in a number of other manual transmissions and have always regarded it as normal.

Edit: Wanted to clarify that mine will pop out of gear before I am moving, if your cars starts to move a decent length and then pops out you may have another issue.

Re-edit: Also wanted to clarify for me it happens only once a month at most, if it happens every time that would also be of more concern.

 

Yes, thats what it does, it feels like it went into gear but as soon as I let the clutch out it pops out...

 

I've had the same '1st gear pop' issue as well. It's happened a total of 3 times, but each time was memorable. All 3 times I was launching on a hill. Twice I was on the incline (launching up a hill), and once on the decline (launching downhill). When I say 'launch', I'm not talking about race level starting, just a gentle 'light turned green and i'm pulling away' launch.

I do need to change out the Tranny juice, I'll let ya know if it happens after that.