You are getting the prompt that the Incident you are trying to save is being used by another user:
You will only ever see this prompt message if you are using 'Optimistic Locking' (Without Warnings) which you set in Object Designer for the relevant Object.
Normally when you get this prompt it means that another User has updated the exact attribute on the Incident that you are attempting to update so the Information that you are currently seeing isn't current. In this case you will need to close the window and relaunch it to refresh the information and you will then be able to update the Incident.
If you however are sure that no other User is looking at the same Incident as you are looking at. For example, if you have just logged the Incident and 2 minutes after this without closing the window you try to make an Assignment and get this prompt message, there is a very small chance that another User would have launch the newly created Incident.
If so, this could be happening due to an Escalation Point performing an automatic action, such as sending a Notification or making an Assignment.
If this is the case the Escalation Point will be looked at as the "other user" and therefore lock the Incident.
The solution to this is to make sure that you don't have any Escalation Point perfoming any action in the time you think the Analyst still will have the Incident open on screen after first logging it. If you have a process of the Analyst logging the Incident and then assigning it, this could very well take a few minutes, so you then wouldn't want to have an Escalation Point doing an automatic action within the first few minutes of the life of the Incident.
This has also been given problem number: 5336. If you are experiencing this issue, please log this as an Incident with your support provider referring to this reference number.