It happened in newspaper jargon for the leading sentence of an article (though they used the spelling "lede" instead), because "lead" was already a metonym for

It happened in newspaper jargon for the leading sentence of an article (though they used the spelling "lede" instead), because "lead" was already a metonym for hot type (which was cast out of the metal). It hasn't happened otherwise presumably because the risk of confusion is normally very low when not in a Pb-filled context.