Running OS X Mojave, wallet version 1.15, the app frequently gets into a state (twice this week) in which simply quitting and then trying to restart results in a crash. When this happens, the only solution is to start fresh from a snapshot.

From the OS X crash reporter, this is found:

Application Specific Information:
Assertion failed: (!setBlockIndexCandidates.empty()), function PruneBlockIndexCandidates, file validation.cpp, line 3265.

I haven't found a way to reproduce the error on demand. There have, however, been other reports of this in both the Veil internal support channel, and on the public side.