When should I overrule a line judge?

Only when it's beyond reasonable doubt that the line judge has clearly made a wrong call (Law 17.5.1 of the BWF Laws of Badminton). Otherwise the line judge's decision on their line is final.

• Your overrule, or the result of a player's challenge where an Instant Review System is in operation, replaces the line judge's original call (BWF Instructions to Technical Officials 8.2).
• Where an Instant Review System is in use, it decides any line call challenge (Law 17.5.2).
• If a line judge is unsighted (hands over eyes), make the call yourself if you can. If you can't, and the Instant Review System can't decide either, call a let (Laws 17.6.6 and 14.2.6).
• Where no line judge is appointed, arrange for the duty to be covered or do it yourself (Law 17.6.5); the service judge may be asked to call the nearest sideline.