I once did a ride about 35 miles from Cambridge to home, average speed was a very erm, gentlemanly 10mph, with Strava recording the ride on my phone. Then once I was home I got straight into the car and zoomed down the A1 to Stevenage to pick something up, forgetting to end the ride on Strava. Boy that second segment was quicker, averaging about 60mph. I didn't know enough about Strava to just chalk off the ride (which I'd have been fine with), and I soon had a message from Strava saying there had been complaints and it was a suspicious ride. I think I eventually worked out how to delete it, and while it's an extreme example you can see how it happens.