Almost a decade ago I deposited a 10k check at the bank and was told the usual (in my case) that it would take x days to clear. After x days and not even seeing a trace of it in online banking I went to the branch.
They were unable to find the deposit or transaction in their system, yikes!
Fortunately I had the little receipt they gave me at the time of the transaction and within half an hour 10k deposit was made available but interestingly the deposit did not come from the check writer but the bank itself. Always curious to me. Never got a clear explanation but I would have been SOL without that transaction receipt!