Refer the link for understanding the concept of Intelligent Search for Material and Vendor in Purchase Order Creation Screen in SAP.
This error occur in case you have miss used values of memory_target and memory_max_target.
To overcome the same follow the following steps
If you want to change the several materials old material group to new material group then follow the following steps :-
Step 1. Find all the materials with old material group either from MM60 or SE16(MARA table)