If the siege weapon gets completed before you finish using your supply., you end up with being stuck until the duration of using supply and cant move anywhere.
This is also same as harvesting nodes which is alraedy used up.
Those actions should not be limited time with a casting bar since they dont have a static time. Sometimes you have 15 supply and it stops at 10.
And with the incoming WvW abilities such as more supply, this may be more problem.
[VcY] Velocity – Gargamell