The user is unable to remove the developer unit (DU) from the imaging kit unit.
If the developer unit is inserted drive side first into the imaging kit unit, it wedges the developer ground contact below the imaging kit ground spring.
The non-drive side (NDS) is usually the first side installed, which has a tab locking feature. When inserting on the NDS first, the DU becomes locked into place and requires a tool to release it.
Refer to this image on how the developer ground contact and the imaging kit ground spring should normally appear.
Note: This issue only occurs on 40K Imaging Kits.