The problem is that you then can't place the lumbermill in the same location again. Let's say you queue up a number of buildings:
- Workshop
- Lumbermill
- Barracks
Then before you finish the Workshop you want to build a Pioneer.
There doesn't appear to be a way of re-sorting the construction queue (maybe there should be) so you need to remove the Lumbermill and Barracks to have the Pioneer built next. Then you can no longer place the Lumbermill back where you originally had it which may or may not be a problem.
