Error occurred in deployment step 'Activate Features': The object has been updated by another user since it was last fetched.
I discovered the problem was caused because I:
- Assigned the Event Receiver to a base content type (CT 1).
- Immediately after attempted to add the Event Receiver to a content type (CT 2) derived from the base content type (CT 1)
To solve the issue, I did not do step (1) above and just did step (2).