Many DDO methods are not re-entrant. Once a major Data-Dictionary operation (save, find, delete, clear) has begun you cannot start another major operation. The reentrancy restriction applies within DDO structures and across DDO structures.
The messages that are not reentrant are:
For example, you cannot send the Find message in the middle of a Request_Save process (even if the message is being sent to another DDO).
When these types messages are sent in a reentrant state, the normal behavior of the DDO is to ignore them. This can create a problem. If you are sending these messages, you are probably doing so in error. Rather than performing whatever action you are expecting, the message is being silently ignored. As an aid the development, the Data Dictionary Inspector provides a way for reentrancy conditions to be reported as errors during testing.