Ctrl-click the "master" key in a group where children keys are partly selected will toggle the children (instead of deselecting them all as one would expect)

Open animation window for cube
In dope sheet, expand the position curve
Ctrl select the keys for x and z only, in the middle keyframe
Ctrl select the "master" key for the entire position keyframe

--> x and z get selected, but y gets unexpectedly selected