Once harm has been done, even a fool understands it.
Context / explanation
from "The Iliad"
Last words are for fools who haven't said enough.Karl Marx
You do not really understand something unless you can explain it to your grandmother.Albert Einstein
In youth we learn; in age we understand.Marie von Ebner-Eschenbach
A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.Douglas Adams