Is there a word for a sentence that becomes false for having been said?

There exist some statements that become untrue simple because someone has said them publicly. A conceptual example might be I have never said the word: contumacious. A more realistic example might be 0QlmIfMjYFLUrCtBT8qd is a secure password.

These kinds of statements are probably not common, but is there any single word or phrase that can be used to describe them?

Solution 1:

Such statements are called self-defeating.

Self-refuting ideas or self-defeating ideas are ideas or statements whose falsehood is a logical consequence of the act or situation of holding them to be true. - wikipedia

A more general concept is defeasibility, where a defeasor 'defeats' a position (not necessarily its own). This is formalised in defeasible logic.