* You need to have experienced SAP *technicians* on board. It is far too big and complex to learn as required.

  * In every case one of the Big 5 consulting firms were involved. Learn to work effectively with them. Be mindful of the fact that they might see you as a competitor that cuts into *their* billable hours.

  * SAP documentation is horrendous. It is massive. Often misleading. Version and configuration details can easily lead you astray.

  * Customization to client's requirements is the norm. These in turn are typically even worse documented (if at all).

  * As much as you might wish to avoid it, you need to work with Oracle, DB2, etc database vendors too.

  * The hardware configurations are often bizarre.  Especially when deployed in clusters with fall-back, etc.