Old circular firing-squad gag

The "old circular firing-squad gag" was how Dalek Caan described (PROSE: Dalek Caan) the tactic in which someone forced surrounding enemies to fire on each other.

History
The Daleks were tricked into firing upon each other because of the relocation of the planet Gallifrey at the end of the Last Great Time War by all incarnations of the Doctor and the TARDIS. As the planet had been subject to orbital bombardment by the assembled Dalek Fleet, its sudden disappearance perceived by the Daleks in their slower gradient of time, caused them to be inadvertently caught in their own crossfire, resulting in the destruction of the vast majority of the Dalek forces and rendering their species near-extinct. (TV: The Day of the Doctor, PROSE: The Day of the Daleks)

While other Dalek survivors escaped elsewhere, (PROSE: Dalek, TV: Doomsday) the Dalek Emperor would explicitly recall that his ship survived the Doctor's "inferno", albeit crippled, before falling through time and emerging in the post-Time War universe. (TV: The Parting of the Ways) Dalek Caan, who successfully bypassed the time lock protecting the war's events to enter the first year of the Time War, saw the event among many others as he was exposed to time. (PROSE: Dalek Caan)

In a variant of the tactic, Nick, when faced with one Dalek Executioner in front of him and another behind him, ducked as they opened fire, resulting in the two Daleks destroying themselves. (TV: Eve of the Daleks)

The tactic was employed during the Master's Dalek Plan when an AI hologram in the form of the Fugitive Doctor was surrounded by a group of CyberMasters including the Cyber-Leader and its subordinate. Not realising that she was a hologram, the CyberMaster opend fire and inadvertently killed each other, with the resulting energy which would have regenerated them being used to reverse the Thirteenth Doctor's forced regeneration into. (TV: The Power of the Doctor)