It's all about managing expectations to some extent, and not creating unrealistic ones. It is very difficult in some instances though, when you're being told by a supplier one lead time on a supply of components, and you know from previous history, they may or may not keep to that. I routinely add "extra" time to the delivery dates I give clients when I know we may get hit like that with certain suppliers, but even then, I know I've been caught out before now.
But you do need to keep the lines of communication open, even if it's just to appologise and say there's no news yet. It's very easy for a small supplier/manufacturer/dealer with lots going on and lots coming in and going out to have that sudden, here, weren't we expecting such and such moment, a few days after the event. Whereas the customer has been sat focussing on just that single item since it was ordered.