The list of biblionumbers in a user's cart is stored in a cookie, and browsers all have a cookie size limit. If a user tries to put too many items into a cart there might be errors, including losing the contents of the cart altogether. We should warn the user (both in the OPAC and staff client) if they have reached that limit.
Following tool gives information about cookie limits in different browsers: http://browsercookielimits.squawky.net/