Why does a validated sample fail in mass production?

A question I hear often:
“The sample was fine… why do problems show up in mass production?”
Have you seen this happen in your projects?
At prototype stage, everything looks stable:
small batch, controlled process, experienced handling.
So the sample passes.
But here is the real question:
Does a successful sample really represent production reality?

When scaling up, what usually changes first in your experience?
Material variation?
Drying consistency?
Mold stability?
Assembly variation?
Or process drift over time?

For transparent parts, small changes become visible immediately.
Have you noticed this?
-stress whitening
-flow marks
-black specks
-clarity variation
-cracking after assembly or use
But the key question is:
Are these material issues, or system variations finally becoming visible?

A sample answers one thing:
Can it work once?
Mass production asks something else:
Can it work consistently at scale?

In your experience, what causes the biggest gap?
Material?
Tooling?
Process?
Or real-use conditions not fully considered in early stages?
Curious to hear different perspectives.