Nick:
The large denomination chip was hidden behind something and DID NOT get pushed forward or if it was pushed forward it was hidden behind a stack or two of smaller denomination chips, in either event it was not visible.
Everyone:
There are two errors here, one by the all-in player, but not having his large chip visible and two by the caller, in not asking for verification of the amount of the all-in.
We have had similar discussions on this forum, mostly dealing with errors in counting the amount of an all-in stack, and it was generally agreed that if the amount 'concealed' was 10% or less than the 'disclosed' or unhidden amount there isn't any reason not to require the all-in player to place that chip in play and to require the calling player to put that additional amount into the pot. I would think that a similar concept might be applied to this situation. For example, if the original all-in was declared to be 50,000 chips and the hidden chip was 5,000, I find it hard to believe that the calling player would not call that additional amount. Now if the all-in was alleged to be 30,000 and the hidden chip was 25,000, I can certainly understand why the calling player might not want to call the additional amount.
There are a lot of other factors that need to be considered before I would be willing to make a recommendation, some of them being:
1. Do these players know each other?
2. Is there ANY possibility of collusion, ie., chip dumping?
3. What is the experience level of the players involved?
4. Was the large chip hidden by mistake or purposefully, etc., etc., etc.