Bad elevation data (either inflated or inaccurate)
Unrealistic times for the KOM/QOM and top positions in the Leaderboard
Unfavorable start and endpoints
Original segment data has errors or GPS drift
Segment is Hazardous or has hazardous aspects
Bad elevation data exists if the data from which the segment was created (the original ride or run) had bad elevation data. Sometimes, categorized climbs are auto-generated by Strava if the area has no prior Strava activity - this can lead to auto-generated climbs based on bad elevation data that is either unrealistic or incorrect. If you are the creator of this segment, you can delete the segment. If you are not the creator, the best thing to do is to "Hide" the segment on your ride or run. See Hide/Unhide Segments.
False times can be posted on the Segment Leaderboards if GPS errors occur, or people leave their devices on while driving a car. If you see unrealistic times posted for a Segment, where it looks like bad GPS data or data recorded while driving a car, you can flag the offending ride. For instructions on how to flag a ride, see False KOM/QOM.
Unfavorable start or endpoints can greatly affect whether you will match the segment. When the segment was created using data that does not represent the correct route, it is most likely due to GPS drift which occurs when the recorded coordinates drift away from the actual path taken. For tips on how to prevent unfavorable start and endpoints in segments, see Create a Segment. Other unfavorable start or endpoints include intersections and stop signs. Again, if you are the segment creator you can edit the segment to improve the start and endpoints. If you are not, you can create a duplicate segment that includes better data, then "Hide" the former segment.
If the original segment data has errors or GPS drift, it is problematic for matching to other user's rides, and may provide inaccurate data. It is best to try and recreate these segment using better, more accurate data.
If a segment has a hazardous feature, you can flag the segment as hazardous and remove all competition with the Segment Leaderboard. For more information, see Flagging a Segment as Hazardous.