AutoCAD uses anonymous blocks for supporting hatch patterns and dimensions. These types of blocks are also created using AutoLISP or similar means, but cannot be intentionally created in the drawing editor. Also, anonymous blocks cannot be modified using AutoCAD editing commands.
Anonymous blocks can pose problems when extracting block information for compiling data, such as in a spreadsheet. For example, if you needed to know how many blocks of a particular item were inserted in a drawing, the number of blocks may be incorrect if the block became an anonymous block at one time as a result of using an AutoLISP routine.
Although you cannot prevent anonymous blocks from being created, it is important to know that they exist.