Trackside tyre barriers / stacks at Estoril are a fine example. Someone runs into one of those and push them on the track. Thats's fine BUT the tyres remain there for some time and there's no warning such as yellow flags about them. It's nice to push through a corner and suddenly you run into a stack of tyres in the middle of the racing line as you're unaware someone 30 secs ahead of you had hit them. There should be a system in place that would call for yellow flags if hard objects were on the track. I don't mind about the orange cones but tyre stacks are a different thing.