https://arstechnica.com/tech-policy/2020/05/twitters-first-fact-check-on-president-trump-calls-out-false-claims/

Twitter's newest fact-checking initiative, which slaps warnings on misleading posts by major public officials, appeared on arguably the biggest possible account in North America on Tuesday: President Donald Trump.

Earlier that day, Trump used Twitter to allege that mail-in voting is inherently "fraudulent." Hours later, his posts were updated by Twitter to include a clickable, plain-text notice—"get the facts about mail-in ballots"—next to an exclamation-point icon.