I have an issue where my camera loses it's connection to the lens when using my 600mm f4 IS II with teleconverters under a specific shooting scenario. With the teleconverter mounted (either 1.4x III or 2x III) there is a slight amount of play in the connection. The easiest way to replicate the issue is to place the lens on a tripod and aim it up slightly, rotate it to portrait mode, initiate the IS, and then lightly pull back on the upper part of the body. When doing so the IS will stop with a sudden "clunk" and if you are looking through the viewfinder you will see the image make a sudden and significant shift. Once you stop pulling back on the body the IS will kick back in and normal lens operation resumes. It seems pretty clear to me that the slight amount of play is causing one or more of the contacts to separate and the lens is losing communication with the camera or losing power. With the camera in landscape orientation it isn't an issue
I've tried this with two copies of the 1.4x III and one copy of the 2x III and it has happened with all of them. I've also tried different bodies and it's the same for all of them (1DXII, 1DX, 7D2, 1DsIII). If I mount a body directly to the lens without using a teleconverter the issue goes away. Also, using the teleconverters on other lenses I own does not generate the same issue, it's only with the 600mm. Has anyone else experienced this, or would a few people be willing to test it on their own lenses? I had this happen a couple times under field conditions but once I figured out what was happening it is pretty easy to work around, if it is an issue that is unique to my lens though I will have Canon take a look at it.