How can a helicopter show up at 29000ft, 547kts at Flightradar24?
@mins answer contains good information, however the conclusion that the mistake is in the Flightradar 24 database is not correct. 

It is not Flightradar 24 that has a database error:


  30040D IS I-PTFT it is NOT I-MOFI F2TH I-MOFI is miscoded on one box
  and using 30040D incorrectly. I-MOFI is 30047D please change it back!
  Why was it changed in the DB when I-MOFI was correct in the DB, I
  thought miscodes were not allowed anymore?


What this says, is that the 24 bit Aircraft Address (a.k.a. the Mode S address) 30040D belongs to the aircraft with the registration I-PTFT. 

The registration I-MOFI has the 24 bit address 30047D, but the quote says it is miscoded on one box (transponder). 

Flightradar 24 receives the Aircraft Address with the ADS-B messages from the aircraft and uses the database to find the aircraft associated with the address. 

From the screenshot it can be seen that the address they receive is 30040D. In their database, this is correctly linked to the I-PTFT helicopter. 



The fault is with the aircraft with registration I-MOFI; it transmits the 24-bit address 30040D while it should have transmitted 30047D. The transponder has the wrong 24-bit address encoded into it.