1. Play a multiplayer game running a mission such as Invade and Annex or Insurgency which provides for medic roles.
2. Slot in as a medic.
3. Ensure there are med packs and a medical kit in your loadout.
4. Go into the field and try to revive downed players.
5. Eventually you will run into someone that cannot be revived.

If you gather a significantly sized team of testers together and have them walk recklessly into the face of an enemy squad or tank or lfrit or something else deadly, without attempting to survive, such that they all die nearly immediately, it will likely be very easy to reproduce it quickly.

Sometimes one medic can revive while another cannot. Sometimes the downed player that cannot be revived can be dragged and carried, usually this is not the case. I have never seen the case where you can drag but not carry or carry but not drag. Sometimes dragging or carrying the downed player to a different location will enable the revive option...however this is rare. Most of the time if you cannot revive the guy, you also cannot carry or drag him. Sometimes when you cannot revive or drag or carry some other player can, and then you can occasionally get the revive/drag/carry options to appear if they do so. The bug seems to me extremely sporadic but quite common indeed.