The question remains what happens if the apertur actuator on the body is caught (as you described) and the DOF-previw button is dis-engaged.
I assume the body works normal. So there would be no sign of warning about mal-coupling of the lens. Except for that little red dot in the mirror box.
Thus one might have engaged the DOF-button, changed lenses "blind" witgh button still engaged, then realizing anbout the error, die-engaging the button and thinking everything is. That is not (no harm, but malfunction).
As general rule anyone grabbing for a Canon out of the FD-range with DOF-button with the lens fitted and not knowing about the recent history of handling (eg. others had access to the camera) should either:
-) take off the lens and before setting it on again check for the aperture actuator on the body to be in neutral position,
-) keep the lens on, set the aperture dial on the lens to a small aperture, engage the DOF-button and check in the viewfinder on the effect, then set both in neutral- resp. A-stand again.
(I hope not to have deviated too much from the OP's question as this of interest to anybody new to the FD-system.)