Honda V4 bulletin VF750 #4 warns about this. The valve adjustment locknuts can unscrew on 1982 through 1984 VF750s. What makes them do this despite adequate and reasonable tightness is the as-forged surface finish of the followers (rocker arms). To combat this Honda issued the bulletin and made available a special extra-long 10mm box wrench. The locknut that put this hole in the valve cover also put a nasty crack in the cylinder head. Check out the bulletin. What it doesn't say is that in 1985 the followers were redesigned by having milled surfaces under the locknuts.

Isn't this something. Same bike, same engine, same incident, same cause, the high velocity rocker arm locknut. This is the front cylinder head, the mate to the valve cover pictured above.