Uncaught third strike

In an infamous example, on September 28, 2019, Kole Calhoun reached base on Justin Verlander's 3000th career strikeout, and then scored on Andrelton Simmons's home run immediately thereafter.

This has happened once in MLB history, as on May 5, 2021, John Means of the Baltimore Orioles threw a no-hitter against the Seattle Mariners, striking out 12 batters while facing the minimum of 27 batters; his only baserunner, Sam Haggerty, reached on a dropped third strike in the third inning by catcher Pedro Severino and was subsequently caught trying to steal second base.

A 1796 German book on recreational games for youth contained a chapter on "English Base-ball" which describes a similar rule.

There were no fielding gloves, and errors were common, so the batter's chance of reaching first base safely was not negligible.

[2] This rule continues in baseball today, but catcher became a required position, and starting in the mid-19th century, catchers began moving up to their current position directly behind home plate so as to catch pitches (now following a flatter trajectory) directly on the fly.

This is why the catcher is given credit on the putout in strikeouts, as with any fielder who catches a ball in play on the fly.