These footballers in South Africa had a very lucky escape here several years ago!

https://www.youtube.com/watch?v=MNJRPOltifI

Just goes to show how dangerous thunderstorms can be, especially to outdoor sports events.

That was very dangerous.. Remind me of strikes seen back in 1965 whilst in Swaziland for 9 months.... 

I was at a rather scary incident at a music festival when a violent squally thunderstorm swept through the site with little warning. A large number of people attempted to find shelter from lashing rain laced with hail, and then a flash and rumble was very close by.
This was a small event , imagine something similar when a very large crowd was assembled such as Glastonbury.

It would be a difficult decision, but you could imagine a situation where it would be best to pause the event for an hour if a storm was expected to cross the site. At least this would reduce congestion if people were more distributed in their own tents and vehicles.

https://www.youtube.com/watch?v=Fe5HNtfTdGE

https://www.youtube.com/watch?v=drSt76O_Kyw 

The most recent example I can think of where a football match was affected by a storm was the England v Honduras friendly in Miami back in early June, a few days before the start of the World Cup. Ukraine v France match at Euro 2012 was another one. IIRC the storm there started just as they were playing the national anthems and very quickly turned into a rather violent storm and then game was suspended for about an hour until it had cleared.

One of my cousins went to the 1990 World Cup in Italy and he was present at a game during which a storm occurred. It was the West Germany v UAE (United Arab Emirates) game at the San Siro in Milan. I remember watching the game on ITV and noticing how dark and ominous the sky looked during the national anthems before the game got underway. About halfway through the first half there were a number of loud rumbles of thunder and the cameras picked up a fairly bright flash of lightning while they were following the action on the pitch, There was then a torrential downpour which lasted for about 15 minutes or so, but I think it had abated somewhat by half-time. It was just as well the San Siro is well protected by the roof, otherwise they would probably have had to stop play for a while if it had worsened.