The major rule that is associated with implementing an ERP system is not to customise it. The risks with customisation are that changes in one area can have a flow through effect into other areas. Regression testing is required which can be extensive, costly and not fool proof. Customisation delays the system going live which can cost the company and cause heightened resistance from end-users. Customisation is likely to void vendor agreements and this will mean that the vendor may be unwilling to correct problems. Upgrades to the ERP system will mean that the customised solutions will need to be reimplemented. This can be costly where there are continued upgrades. Change in personnel can make these implementations difficult. In short customisation creates excessive additional costs.