Show us screenshots that includes your Timeline panel with the relevant properties and keyframes exposed, along with the Composition panel showing the rendered result.
In addition to what Todd asks, I'd like to know what version of AE you're running. Exact version number, please. 11.0.3? 12.1.0? etc.
I have cs 6, version 11.0.
I started up my AE to take a screenshot after taking a few hours break,
and I previewed where the error was when I quit, and it has.. fixed itself.
This has done this to me five or six times,
so I will continue to work now and post a screenshot when (not if) it does it to me again.
Thanx for the quick replies, guys!
Make sure you are understanding how the CTI relates to the frames. View the timeline at the frame level and you'll see that if you move the CTI to frame 10 and then set the out point of the layer to the CTI position the end of the layer extends to the start of frame 11. If you put a keyframe at frame 11 the value of that keyframe will be applied after the video ends.
This shows the right way to place a keyframe on the last frame of the video for opacity set to 0.