Yes, there is a daily purchase limit on debit cards in addition to ATM withdrawals. This can cause a decline when a merchant tries to process a transaction over the customer daily limit. The customer may know they have the money in the bank but are not aware of purchase restrictions. Although merchants have no way of knowing reasons for a decline, if you sell big ticket items, this is something you can alert cashiers to look for.
The daily limits vary by the card issuing bank and the person issued to. For example, Wachovia Bank, a division of Wells Fargo Bank N.A. has issued cards in 2011 with a daily cash withdrawal limit of $500 and daily cash purchase of $7500. In other cases, the limits are much less.