Sometimes - tho not always, video tracks seem to be one frame short. This is most easily visible upon re-opening your project. A clip that was 60 frames long, suddenly turns into 59 in your timeline. 
This seems to happen during initial editing. Sometimes, jumping to the last frame of a clip renders it black. Then if you save your project and re-open it, you see that the last frame actually disappeared.

Reproducible: Sometimes

Steps to Reproduce:
1. Make a project with at least two video tracks.
2. Create a color clip and a title clip. But you can also use a video if you want. It works with all sorts of visual content. Make sure both have the same length. Let's say 60 frames.
3. Put the color clip on the lowest track and place the title clip above it - blending them with the Affine transition.
4. Save the project, close kdenlive and re-open your project.

Actual Results:  
Opening the project again may show different results:
1. The title clip suddenly is only 59 frame long
2. The color clip is only 59 frames long
3. Both clips are only 59 frames long

Expected Results:  
All clips should be 60 frames long!

This also happens with video clips. Audio clips and transitions seem to be unaffected by this. Usually you can fix this by simply dragging the clips to their intended length, save the project again and upon re-opening the bug is USUALLY fixed and stays that way.