You’ve had it happen to you.
After a lifetime of waiting to see Hagia Sophia, Big Ben or St. Mark’s in Venice, you’re finally there.
The facade is swathed in scaffolding and screened by anything from ugly netting to a trompe l’oeil rendering of what the building should look like.
For example, this view of Milan cathedral in 2006:
Scaffolding Watch is your resource for knowing in advance which monuments, buildings and important sites are likely to have obstructed views during your visit.