Harold,
I feel your pain but trust me, you are not alone.
Probably the worst of the many I have had was the steel work in the gardens of Oprah Winfrey's exclusive school for girls in South Africa. We cut all the main posts for the walkway arbours about 400mm short and only realised it after they were planted in the ground. As luck would have it, the landscapers decided they wanted to raise the pathways and apologetically asked me if I could please lift the height of all the already planted steelwork by 500mm for which they were happy to pay premium rates. We pulled up all the posts, added 900mm, replaced them and everyone was happy.
The old addage, measure twice, cut once comes to mind.
Kevan