Hi,
The most common reason for inconsistent results is from cases where a runner has used the "GPS Track Upload" tool to create a result for themselves from a track. See:
http://p.fne.com.au/#/gpsuploadThis tools gives the runner a list of controls that they went near... but it often has repeats of S1 and F1. (and maybe other controls too).
This upsets any determination of time and classifier (MP, OK etc).
Sometimes you will see S1 and F1 listed in the list of controls in the results... this is a clear indicator of this problem occurring. You didn't mention which event you saw the strange result in, but an example of extra S1 and F1 is shown in the results for: GERST MOL2021 MS PXAC
It is always recommended that the Administrator for the event reviews results for their events and deletes or corrects any spurious results.
Peter