If a site shows me a special offer (e.g. an extra 10% off) when I'm about to abandon my cart, that's OK with me. Just like if I'm walking out of a store because I didn't get an acceptable price, and the owner calls to me as I'm passing through the doorway.
If the alert says something untrue or misleading (last day of sale, virus on computer, 1000th visitor) that's deceitful.