Multiple Opencv versions in same project

Why having multiple versions of Opencv in same project results in certain functions not working properly ? I had this experience when I used a library that includes an old version of Opencv , adding a new one (to use dnn for example) resulted in [cv::findcontours]( not working properly.

yeah do not do any of that.

no library is supposed to work when you mix different versions of the parts of the same library.

OpenCV is no different.

you should stop this, or at least explain why there is absolutely no way around this very unusual and forbidden version mixing you do.