In case of thunderstorms in close proximity, the tournament must be interrupted immediately regardless of rain (Rule 12.3).; When interruption is ordered it is immediate, unless a ball already past the border line can be completed, in which case the player may choose to complete or interrupt. The...
Source: Rule 12.3, Rule 12.4, Rule 12.2, Rule 16.9, Rule 11.7, Rule 13.1 — WMF rulebook 2.3 Worldwide international sport rules
Why people argue about this
People often assume that if a ball is already past the border line, players can choose whether to finish their turn or interrupt early due to safety concerns. However, the actual rule states they must complete their current shot before deciding on an interruption, which leads to confusion and arguments about when exactly they should stop play.
