I was watching the online coverage of the 5th game in the 2010 World Championship Match when the amazing news came through that all the lights in the playing hall had gone out. There were also reports that the whole building had lost electricity meaning that there was difficulty in entering and exiting the building. Apparently this state of affairs remained for about 15 minutes before power was returned.
It seems that the organisers at least were able to cope with this eventuality, as the game continued with emergency lighting, although the online coverage of the game seemed to be interrupted. Nonetheless such an incident can be disturbing to both players. A similar incident occurred during the Fischer Petrosian match in 1971, although surprisingly Fischer (who was always touchy about lighting) allowed his clock to be restarted in the semi-darkness as he was still thinking about the position.
Interestingly it is countries who have a poorer electricity infrastructure are better able to cope with such a situation. In last years Solomon Islands International, Honiara was hit with a city wide blackout, plunging the 3rd round of the tournament into darkness. No problems, out came the battery powered lamps, and the only game in progress continued. It however did have one practical effect. Brian Jones played 34.Nxe7# but failed to realise it was mate as he did not spot his own bishop on c3 covering the dark(end) squares!
Jones,B (2065) - Tangaibasa,Brandon [A43]
Solomon Islands Int Honiara SOL (3), 25.09.2009
1.d4 c5 2.d5 d6 3.c4 Bf5 4.Nc3 Nd7 5.e4 Bg6 6.f4 Ngf6 7.Bd3 h6 8.Nf3 Bh7 9.0-0 g6 10.e5 dxe5 11.fxe5 Ng4 12.e6 fxe6 13.dxe6 Nde5 14.Qa4+ Nc6 15.Be4 Qc8 16.Bd5 Bg7 17.h3 Nf6 18.Be3 0-0 19.Bxc5 Qc7 20.Rae1 Nh5 21.Nb5 Qd8 22.Rd1 Qa5 23.Qxa5 Nxa5 24.Bxe7 Rfc8 25.Bb4 Nc6 26.e7+ Kh8 27.Bxc6 bxc6 28.Nd6 Rcb8 29.Nf7+ Kg8 30.Nd8 Nf6 31.Bc3 Ne8 32.Ne5 Bf6 33.Nexc6 Bxe7 (D)