Many of us are used to rendering dates on checks and other documents in a MM/DD/YY format, omitting the first two digits of the year for brevity's sake. After all, the chances of confusion are small: It's highly unlikely that a document created in 2019 could be mistaken for something that originated a century earlier, nor that anyone would expect it to still be in effect in 2119.
The once-in-a-century occurrence of calendar years in which the first two digits and the last two digits are identical (e.g., 1919, 2020) might pose something of an issue for this format, however, as noted in social media warnings widely circulated in the opening days of 2020:
Such warnings cautioned that simply rendering the year on a check as "20" (e.g., 1/3/20), for example, could facilitate someone else's later appending "21" to it (e.g., 1/3/2021), thereby converting an uncashed check that had expired due to age into something seemingly current and usable. Similarly, a legal agreement intended to begin on the date "1/3/20" could be altered to read "1/3/2019," thereby making it appear the contractual terms had been in effect much earlier than they actually were.
Many critics found the subject of these warnings rather unlikely, however, noting that many documents are vulnerable to fraudulent alteration by a variety of means regardless of how any dates they include might be rendered. And in such cases, various forms of evidence can usually be marshaled to prove the alteration and thwart its purpose.
Others believe the maxim about "an ounce of prevention is worth a pound of cure" still holds true: scammers generally seek easy targets, so any simple action that can potentially discourage their efforts — including writing out the full date — will usually be easier and more effective than trying to undo damage after the fact.
Whether to follow this advice is an exercise left to the reader: It may not help much (if at all), but neither will it hurt.