Cool. I wonder what's happening with KLM 7021: http://i.imgur.com/ulwdC.png -- it seems to have taken off Amsterdam, then flew to the coast of Norway where it flew in circles.. then decreased the height while flying to Billund, Denmark, then finally returning to Amsterdam.
My guess is that someone forgot to change the flight number on the transponder, so what you're looking at is the path of one plane over a couple flights.
I'm tempted to suggest that the aircraft was put into a holding pattern, likely due to foul weather at the destination, and eventually forced to reroute.
Could the circling be due to waiting for landing due to a lot of congestion at the airport? I'm not a pilot so I'm curious how that's normally handled.
Congestion is often handled via "stacking" aircraft in a pattern like you see in the image, separated for safety and timing of approach. There may well be other aircraft in the stack but not reported via that site or there may be just the one at that time. In either case, the apparent reroute suggests more than congestion.
(For the record: I am not a professional pilot but I have flown and have family who are commercial aviators going on some 40 years.)
http://www.radarvirtuel.com/ is a similar concept but they seem to have more ADS-B receivers in play, so there's better coverage of Europe (and also some of the rest of the world).
My guess is that someone forgot to change the flight number on the transponder, so what you're looking at is the path of one plane over a couple flights.
ADS-B data isn't linked to the transponder code - each plane has a serial number assigned by the ICAO. So this is the simply one plane flying several different routes.
There are multi-leg flights that share the same flight number. I recall of a particular AA flight that has the same flight number from JFK-ORD and subsequently from ORD-MIA (or something like that).
This seems to be a little flakey. I tracked one flight due to end up at aberdeen, and decided to open the page on a different machine so I could watch it out of the corner of my eye. That flight was simply not there. Then, refreshing the screen, it disappeared as well.
