MMB clicking on a track in the sequencer will copy/paste your current frame to that time, but that action doesn't recognize snapping (it'll end up on a subframe)

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• make a new level sequence
• make sure that time snapping is on
• pull a control rig into that sequencer
• set a key on the first frame
•  go forward in time and make a clearly different pose (eg. bend the character at the waist)
• to back to the first frame
• middle mouse on a frame in the control rig track in the sequencer some frames after the second pose you made
• go to that time
• note that:
  • the pose from the frame you were on (first frame) has been copied to this location
    • awesome
  • the time that this key was set is literally where you clicked and as such isn't snapped to a whole frame even with time snapping on
    • not as awesome