Short description of error
It is not possible to insert keyframes between existing keyframes using any method, in any relevant editor like the GE or DS
Exact steps for others to reproduce the error
1- Animate an object's location and rotation with at least 2 keyframes
2- Scrub to a position between 2 keyframes
3- Hit k in 3D view and choose locrot, nothing...
4- Go into the graph editor and do the same in 2 and hit i and choose a valid method... Nada...
5- Go into the Dope Sheet and do the same in 2 and hit i... Zilch...