Actually over subscription is more likely.
If, say, all the pilot slots further up the line are spoken for and there is a significant number on hold then Stop the Pilots.
Ditto all the other branches.
There is a risk (slight) that you will wander off and join something else rather than be bound to HM. Also it is quite likely that you may be given the shilling and sent off to do some adventure training. Just a guess this.