Canon's manual notes:
* If you continue to shoot movies for a prolonged period, the camera's internal temperature will increase. This can degrade the image quality. While not shooting, turn off the power.
* If you shoot under direct sunlight or high-temperature conditions, the (high internal temperature warning) may appear on the screen. If you keep shooting a movie with the warning icon displayed, the image quality might degrade.
* If the icon is displayed and you keep shooting movies until the camera's internal temperature increases further, the movie shooting will stop automatically.
Now it appears that these users were using the camera for an "extended" period:
- I shot a time lapse of the 7D aimed at a clock with my EX1 and the results were about 30 minutes before the dreaded 'red thermometer' began flashing.
- It was quite hot and I was shooting intermittently for about 2 hours all at 720p/60 for slow-mo with my production 7D. I'm sad to say it gave me temperature warnings almost the entire shoot and stopped me shooting at all at least 3 times during the period.
- I just tried 480p60 indoors, 74 deg. F, got to 25:30 before the temp. warning.
I'm using the word "extended" advisedly; for casual users 20 minutes of video is a lot, for serious shooters, 20 minutes isn't long at all.
Some 5D owners have chimed in that they have hardly ever encountered this warning with their cameras, leading some to believe that the 7D is more susceptible to the problem than the 5D.
Interestingly, I remember when the 5D was first announced, there were questions about how the camera would handle extended shooting, and some thought that the 12 minute limit of a sequence was there because of that concern, though a representative from Canon later denied that.
This all poses a dilemma for budget film makers who liked the 5D, but wanted 24p shooting. The 7D seemed to offer a solution to that, but if it doesn't handle extended use/shooting situations, then it may be a bad choice.