Wikipedia defines it quite nicely:
“In retail sales, a bait and switch is a form of fraud in which the party putting forth the fraud lures in customers by advertising a product or service at an unprofitably low price, then reveals to potential customers that the advertised good is not available but that a substitute is. This use of this term has extended to similar situations outside of the marketing sense.”
The world is far too transparent now, people will expose these techniques faster than lightening. However if you’re reading this you probably don’t have this attitude, however I do urge, to check how you manage expectations – you may be doing this… unintentionally.