You are working on group of companies. They don't want to share any data between companies, simply no integration required, therefore mgt wants to have one client for one company that end up having multi-clients in prod instance. However, one of the SAP local guy told you not to continue with this lanscape. 

Some of the potential problem of using multi-clients in one prod instance are: 

1. problems affecting one client immediately affect all other clients. 
an eg.: 1 client runs a job that fills up a tablespace or file-system. 

2. a system problem (system crash) affects all clients immediately. e.g. an Oracle archive stuck will affect all clients. 

3. programs/tables are client independant. Invidual customers cannot make changes to common programs/client without affecting the others. 

4. Poorly written ABAP's will cause bad response throughout the SAP system, affecting all clients. I shudder to think of a situation where the programmer for 1 customer stuffs up and the other customers demand blood! 

5. Taking all of this into account, your change management will turn into a NIGHTMARE! especially considering that each customer probably does no care about the other customers, so EVERY change of theirs is the most important one. 

The above are some of the problems if you have multi-client in one SAP instance and there are many more arguments. 

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