Cartoon by Don Mayne www.researchcartoons.com
There are valid circumstances where withdrawal of consent will be impossible (e.g. if participants are completing an online survey without personal identifiers, and an individual submits their response there is no way they can withdraw their consent and data later) and sometimes withdrawal might be unsafe (e.g. a project might involve a treatment regime where discontinuing before the conclusion is unsafe). If such circumstances exist this must be made clear to potential participants upfront – NOT when you are adding seasoning to the boiling cauldron.