Does your geometric network contain both Z-enabled and non Z-enabled feature classes? If so, you are most likely encountering a bug:
NIM068617 Editing ArcSDE geometric networks that involve a mix of Z-Enabled and Non Z-Enabled feature classes results in the the following errors: "Modify Feature: An unexpected failure occurred. Shapes of this entity type not allowed in this layer [SHAPE]"
The workaround is to create a geometric network in an ArcSDE feature dataset that contains all Z-enabled feature classes or all non Z-enabled feature classes.