Hi mate,
I had this happen to me while filming a Wedding on the weekend. Exactly as you described. One 12 minute file was recorded and then I was at the end of the second 12 minute file and it started to go in slow motion and then the screen froze with the red record dot in the right hand corner. I turned the camera off and the image was still on the LCD? Then I pulled the battery out and turned the camera back on and I'd lost the file. Two questions
a) Can you recover the file (it's meant to be a mov file but now appears as a zero kb dat file
b) Can you get Canon to fix this. I know that Canon wrote you a polite email back which more or less said that they can't do anything. But if you and I are having this problem is this for us to deal with or for them to fix? My Mark III is still under warranty.
To give you some ideas of what I was shooting with, I had a 70-200 IS on at the time, I'm using Sandisk 32gb 60mbs CF cards and genuine canon batteries. The climate I was shooting in was on the cooler side probably around 20 degrees so I don't think this is an overheating related problem.
I look forward to your reply, Hugh : )
Hey Hugh, I had the problem again today too

. I lost 12 mins of a wedding ceremony on one camera!! I sold all my sandisk cards and switch to 32gb Lexar 1000x But still the same issue, so the problem isn't with the cards! One thing we have in common is all the times it has happen to me I have been using a canon 70-200 IS II lense! maybe this is where the problem lies? Has anyone else with this combo had problems? We really need to get onto this and get canon to fix it ASAP, this is a major fault which renders the camera useless for events if you can not trust it!