When you enter a duct size or offset (elevation) on the Options Bar you may have been bitten by the "old scroll wheel automatico-resizer-syndrome"?? Ouch...you picked a 12" high duct and suddenly you find your height is 48" just because you moved your mouse back over the drawing window and used that middle button/scroll wheel to zoom in or out.
Trouble is the "focus" of Revit is still in that field on the Options Bar. The TAB key, doesn't help. Pressing Enter doesn't help... Nothing seems to...
Well here's what I do, I click on one of the Scroll Bar's buttons to move the focus back into the drawing window. This allows me to move the focus back into the Drawing Window without actually clicking in the Drawing Window, which would place a first point instead of letting me zoom in/out.
Edit: Comment by "Mr. Jackson" suggests his technique: Use the middle button to "pan" for a second. I think is a better solution too!