Not sure if this is a well known issue and it's pretty minor but I don't remember seeing it before and a search didn't turn anything up so thought I'd mention it in case it's somehow related to a recent change.
When building something if you walk away exactly when you put up the building sign the sign persists even though there's no materials in it (unlike how it disappears if you walk away from an empty building sign normally). If you open it again it disappears when you walk away. The building sign persists through going in and out of a mine so it's probably not a client issue.
Having a hard time thinking of how this could be exploited or cause any real issues, I suppose it's possible that it might not decay since the number of materials can't decrease (if that's how partially built things decay) but I haven't had time to test that yet... I suppose if a client was engineered to do this to a row of buildings quickly then a wall could be made for free that would only cost bb, but yeah easily removable by anyone who simply examines them.
*shrugs* Sorry if it's pointless lol