I had one case where a station (not in contest mode) called me using message 2, so he never sent his grid.  As I understand it, WSJT-X is supposed to switch the non-contest-mode station into contest mode when a contest-mode exchange is received, but this combination of events didn't produce a valid contest QSO.  Not sure if there is a software fix.

I think those of us who are interested in VHF contests need to put on a big educational push so that those VHFers who are not particularly interested in contests learn about contest modes and how and when to use them....they only give up a signal report.  I think the same is true of HF digital contesting, but I have spent less time at that and don't have as much of a feel for how many contest-mode compatibility problems exist there.

73,
Steve VE3SMA