Sometimes one needs some kind of "transition priorities" or "firing order" for transitions but as stated in the UML superstructure 2.4.1:
UML Superstructure 2.4.1
In situations were there are conflicting transitions, the selection of which transitions will fire is based in part on an implicit priority. These priorities resolve some transition conflicts, but not all of them.
An alternate approach to this issue is to use junctions to define "priorities". Another advantage of this solution is that it improves the readability of the model.
Here we see an example were the firing of the transitions will have key consequences to flow of the state machine
We would expect a result such as this:
Here the same statemachine without the problem of conflicting transitions and a much more readable model.