It is possible for a user (with developer access) to bypass the workflow transition access level settings in the following case:

Workflow statuses involved in this issue:
NEW New issue
ACKNOWLEDGED Confirmed issue
IN PROGRESS Issue in progress

In the "workflow transitions" page, the following is specified:
Required access level to change status:
NEW -> ACKNOWLEDGED: Manager
ACKNOWLEDGED -> IN PROGRESS: Developer

The workflow transitions are specifed such that the status should go only from NEW -> ACKNOWLEDGED -> IN PROGRESS and not directly from NEW -> IN PROGRESS.
This should mean that the manager always will have to "accept" the issue before it is started.

If, however, on the "Workflow thresholds" page, it is specified that a developer is allowed to assign an issue (which is actually desired), the developer may shortcut the above specified workflow and move the issue directly from NEW to IN PROGRESS.

This should not be allowed when the workflow does not allow such a direct status change. The developer should only be allowed to assign the issue after it has been ACKNOWLEDGED!