Especially if you don't meant it.
The 26-year-old was due to fly from the South Yorkshire airport the following week and tweeted, saying: "You've got a week and a bit to get your s*** together, otherwise I'm blowing the airport sky high."
Police were alerted to the threat and he was arrested under the Terrorism Act on suspicion of conspiring to create a bomb hoax.
So that seems to be how far the UK has gotten with anti-terrorism legislation. You can be arrested and charged for joking about blowing up an airport even if you happen to be nowhere near one at the time.