Time Bar
A contractual deadline for giving notice of a claim, after which the entitlement is lost — regardless of merit. Under FIDIC 2017 the window is 28 days from the event or from awareness of it, and it is enforced as a genuine condition precedent.
The word people underestimate is "regardless". A time bar does not weaken a late claim. It extinguishes it. A contractor can be entirely right about the facts, entirely right about the entitlement, and recover nothing because the notice went in on day 31.
And actual knowledge does not cure it. The other side knowing perfectly well what happened is legally irrelevant — the notice provision exists precisely so that both parties know, definitively, that a clock has started and a claim is live. That certainty is the point, and it would be destroyed if informal awareness counted.
Which makes the most expensive risk on any project the one you correctly predicted, watched materialise, and then failed to notify.