Lars. It's all about checking what you say beforehand with what you submit after.
(Leaving aside n00b issues here - I know you know what it's all about.)
You submit a form before you ride. Assuming it's for a route you're not expecting me to pre-check, I don't care - you submit the form saying, in effect, "I'm going to ride this now/when advised". If I'm lucky, you've put a link to your route on the entry form.
You do the ride, you send me an email. (Rider emails are great - it connects me with those rides that are going on all over the place, and that's brill.) I compare your track with your original intentions, which - if you've submitted a Googlemaps link to your original shortest-distance route, is really easy.
I validate your ride, or email you because I can't on the face of it validate immediately, for whatever reason, and want more info.
ETA: Resuse of a brevet: We've been here before. It's up to the org who sold you the brevet - but speaking personally, I'll allow brevet reuse if:
You DNF/DNS through safety reasons - including weather related reasons such as ice, tornadoes, plague of frogs.
You DNF/DNS through illness - "Once I'd thrown up for the third time Chris, I decided to head home..."
Emails saying "I woke up, and it was dark Chris, so I went back to bed and didn't ride" will result in a "There there" email and the brevet is Most Definitely consumed