I seen a Vw Transporter for sale on Ebay, the owner had a new cam belt fitted without being advised to have the water pump changed at the same time, this is what the eBay seller wrote:
My pride and joy, and a superbly running van for me for 5 years until Spring 2011, when due to a poor service from *********** (cam belt assembly replaced without replacing water-pump) my van suffered a major mechanical failure and hasn’t run since. Inspection revealed that the water pump had seized, causing valve damage and snapping the cam shaft. It is possible that the pistons have been damaged. The engine has been stripped down (see photos) and all components removed are in boxes in the rear of the van. The buyer will have to repair or replace the engine if wishing to run the van.
Some vehicles cam belts run the water pump, which means if the bearing fails in the water pump the cam belt will break causing engine damage, the water pump for the above vehicle would had cost about £30 plus anti-freeze and vat extra, a small price to pay for peace of mind! We always advise our customers to have a water pump replaced at the same time.