For gas powered sprayers, a worn engine belt is just such an issue. Worn belts slip, preventing the engine from driving the pump, or simply break. In either case: downtime, missed appointments, repair expense, lost revenue.
This problem should never happen. A worn belt is totally obvious. It is easy to change and it doesn't cost a lot. Periodically inspect, or train your technicians to inspect, the belt. When it begins to show wear, replace it. Hopefully all your spryayers use the same belt so it is not a problem to keep an extra belt or two in the shop.