Role-play situations: a sceptical CTO, an impossible timeline, a feature you do not have, a losing bid.
3 questions in Presales & Solutioning.
Name the failure out loud immediately, take one narrated attempt at it, then route around it to keep the meeting's purpose intact. Afterwards, send a same-day written note saying what broke and what you are doing, tell the truth about whether it was a defect or a gap, and re-demo on a named date.
Separate what is technically true from what you would prefer, write it as a one-page recommendation with named conditions that would change your view, and escalate to the level that can accept the risk. If overruled, get the risks recorded and stop objecting.
Let them finish, because interrupting a reference confirms the worst reading of it. Then use the call to establish what specifically went wrong, and treat the existing customer's unhappiness as the problem to fix rather than as a sales incident to contain.