IME (3 episodes of a derailler malfunction) all were due to cable problems: two for apparently good plugins of cable ends
that were not, it turned out that there are two 'pops' or 'clicks' when one seats a connector and only the second click
confirms the electrical connection. These two occurred: once when the bike fell over and the R brifter rotated in on the bar
and once when I hit a really bad bump and the R brifter again slid down slightly on the bar. The 3d fail was unrelated to
any incident but an RD stopped working and after an etube eval showed no connection, a cable substitution fixed the
problem. My local Di2 guru has found over the past 8 yrs that a bad connex or occasionally a bad cable is almost always the problem
when a malf occurs.