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Potential problems with satellite as a broadcast source.......
Solar activity
Weather ie snow on dish, very heavy rain
Available bandwidth, many on DS forum stating that now Sky are squeezing HD channels (4 per transponder) quality has dropped
The satellite in use is coming to the end of it's life (pass original life) so more at risk for failure with replacement not due till next year
satellite might just wander out of orbit like many before or be hit by space junk etc
Dish on house having problems silty LNB etc
Potential problems with cable
Cable cut by accident etc
Car hits roadside cabinet or vandalism, easier to fix than the satellite
The above ignore headend problems such as fire as I assume they have disaster recovery plans for this.