If you replace the IPK from a donor car and replace the LSM (Light Switch Module) then it will seemingly work with no errors, apart from showing incorrect mileage. The mileage from the donor car will be shown. It will also increase as per normal if you drive the car. However your original mileage is now only recorded in the EWS (Engine immobiliser (EWS from the German, Elektronik WegfahrSicherung)) and keys, but can not be put back to the IPK without the help of mileage correct software/hardware. The T4 will code a blank IPK but reset the mileage to 000000 on cars that have had ECU (Engine Control Unit)'s swapped out. You will also need a new LSM or one that has been reset by one of the capable members.
It also prevents any dealer options being changed due to VIN (Vehicle Identification Number) mismatches and can cause havoc in general. Coupled with another error I had on the car, I turned a car into a quite expensive brick until I was able to find the fault, which without the T4 working properly took an hour or two.
Please be wary if you have changed ECU's for second hand ones, and make sure the T4 operator is informed. Still unknown why to me my T4 attempted an automatic restore as soon as it was connected! Could have been down to the second error but I don't wish to try again to find out!