An offer needs to integrate tightly with the call-to-action which gets people to it, to ensure that as few as possible end up mistakenly believing that you don't deliver on your promises.
"Bait and switch" involves customers being deliberately offered one thing, even though it's not available and they would have to settle for a poorer alternative. None of us would do this, right? It's a con. But there's always a risk that you could be perceived as doing just that, even when you're not.
How? By mis-matching your call-to-action and what you deliver as a consequence. Suppose you said: "Come and see us on Stand 999 at WidgetEx and collect your free pen", only to run out on the first morning. Subsequent visitors would suspect you never had any free pens in the first place and were untrustworthy. Not true, but the damage is done.
You can give the same impression through lack of clarity on your website. How often do we see something offered, click through but fail to find it? Just because you know that your company catalogue is one click away from the home page on a page cryptically called "downloads", doesn't mean your prospects will make that deduction. Yet how often do we see adverts which say "visit our website to request our latest catalogue" but find no mention of the catalogue when we get to the home page?
An offer needs to integrate tightly with the call-to-action which gets people to it. Ideally, when prospects take action on seeing a promotion, they need to be sent somewhere where they'll see the promotion almost repeated, in terms of text and design. That should ensure that as few as possible end up mistakenly believing that you don't deliver on your promises.