People's accounts have been closed for this. (wanting to reserve space for upgrades (cash or supported), wanting to create last minute I space, using refundable ticket and then cancelling to reserve a specific seat for a regular later non-refundable purchase, ...). UA has the data to discover this and can see patterns. So once will probably not be noticed but patterns will be..

There is very little in the travel game that folks have not tried over the years and that airlines are not aware of. Not all get caught but some do and regret it. And getting banned from a carrier can be a problem difficult to explain to you employer as one such individual related.