Recently I've seen a Cessna 208 Grand Caravan flying as high as 43,000 ft MSL on flightradar24. Perhaps it's still in the air if you look over eastern Canada (the Grand Caravan has a unique shape from the other planes on fr24, looking like a sailplane). The Grand Caravan is unpressurized, does that mean all passengers wore oxygen masks? Or was it just fr24's error to portray a Grand Caravan that high (and if so, how would such error occur)?
The Wikipedia page for the Caravan family lists the service ceiling at 25,000 feet. I think it's very unlikely someone got one (any version) as high as 43,000, and less likely yet on oxygen in an unpressurized cabin.
That means this is probably an error in altitude reporting. How such an error could occur is difficult to say -- I'm not familiar with fr24, so don't know how they obtain altitude data.
Note also that "At 40,000 feet, 100-percent unpressurized oxygen provides the equivalent of ambient air at 10,000 feet." It would be unnecessarily risky to fly higher than this without some kind of pressure support, and would require full mask oxygen, not the cannula often used by private pilots who venture about 12,000.
Flight Radar 24 works mostly by receiving and decoding ADS-B (Automatic Dependent Surveillance - Broadcast) messages from aircraft. In these messages, there is no data on the aircraft type.
To display the information about the aircraft, Flight Radar 24 has to use the 24-bit address (a code similar to a MAC address in networking) that is uniquely assigned to an aircraft, and look up the aircraft information in a table in their database.
What happened here, is likely one of the following:
the aircraft was flying at 43 000 ft, but was transmitting the 24-bit address of another aircraft (this can happen due to various reasons, from a mechanic putting in the wrong 24-bit address, to electric connection failures). The lookup therefore picked the wrong aircraft (a Cessna Caravan) from the table.
the aircraft was flying at 43 000 ft, and was transmitting the correct 24-bit address, but the table was wrong. This can happen, for example, when a new aircraft is registered on an address that previously belonged to another aircraft (a Cessna Caravan in this case), and the table used by FR24 has not yet been updated.
the aircraft was the Cessna Caravan, but the altitude encoder was erroneously encoding 43000 ft instead of the actual altitude. This occasionally happens, but if the aircraft is receiving Air Traffic Service and is within (secondary) radar coverage the discrepancy would be noticed by ATC very quickly. Depending on the airspace class and the controller workload, the flight may either be allowed to continue based on verbal altitude reporting or the pilot would be asked to land or otherwise leave the airspace.
Without more information about the date, time, location and/or aircraft registration of your observation, it is impossible to discern which of the above cases applied here.