Danger

Red was the colour for danger on Earth. Throughout the rest of the universe, the colour for danger was mauve. (TV: The Empty Child)

Babies had the ability to sense danger. (TV: The Girl Who Died)

When the TARDIS landed on Wengrol, the First Doctor could tell that there was danger outside from what he saw on the scanner. (PROSE: The Sons of the Crab)

The White Guardian once warned the Fifth Doctor of danger. (TV: Enlightenment)

According to the Valeyard, the Doctor's companions had been placed in danger twice as often as the Doctor. (TV: Mindwarp)

Gilgamesh liked danger, as with it came the chance to gain glory; it also staved off the boredom of life. (PROSE: Timewyrm: Genesys)

The Thirteenth Doctor said that she did not like people being in danger. (TV: Kerblam)

On 2 April 2005, the new webmaster of Who is Doctor Who? expressed concern if the duplicate of Mickey Smith continued to put people in danger. (PROSE: The Doctor Was Involved in the Dummy Massacre)

Information from invalid sources
Upon entering the Doctor's TARDIS, Data remarked that time travel was "very dangerous", but that it was okay because he knew what he was doing. (NOTVALID: LEGO Dimensions)