SUMMARY
At the moment there is an option in the misc settings which checks if the first clip matches the project's render profile which is unchecked by default.
In my opinion, this behaviour is not sufficient.
First of all, the option should be checked by default (I would remove the possibility to even uncheck it).
Second, it should check if ALL clips on the video tracks/visible do match the project's render profile since the user could use clips recorded in different formats?
I don't know if the option checks clips in the project's bin or on the tracks but it does only have to check the clips on the tracks, I guess.
I am not even sure what the option does exactly. When I have enabled it and set the render profile to progressive although I use interlaced clips only, it didn't warn me or anything (not even when rendering the video). I had one title clip on the track but title clips should definitely be ignored?
Therefore, the third point would be to fix the checking behaviour and warn the user maybe when opening the project and/or when rendering the project.
It should compare interlaced/progressive, the number of frames, the size of the input clips and everything which can not be achieved easily by rendering them in another format (of course everything after applying effects to the clips? maybe too complicated?).

STEPS TO REPRODUCE
1. Create a new project and check the option in kdenlive which checks if the first clip matches the rendering profile
2. Set the rendering profile to a progressive format
3. Add only one clip which is interlaced and move it on the video track
4. Render the project

OBSERVED RESULT
See that there is no warning

EXPECTED RESULT
There should be a warning and you should not have to set the option.
The option should not be available.

SOFTWARE VERSIONS
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION

Related to https://bugs.kde.org/show_bug.cgi?id=395701 which is not a bug but could have been avoided by this behaviour.