During a calibration process, if one of the devices lost its tracking (ie, vive tracker can't see lighthouse), the calibration process will automatically stop.
However, when starting a new calibration, it continues where it left off, using bad sample from the aborted run.

In addition, if the aborted run was running in, say VERY_SLOW 500 samples mode,
then the calibration that is rerun in FAST 100 samples mode,
the existing collected samples could already be more than 100, causing the process to never terminate, requiring app restart.