千虑一失

lit. a wise person reflecting a thousand times can still make a mistake (idiom); fig. nobody is infallible

a thousand tries leads to one success (idiom, humble expr.); Even without any notable ability on my part, I may still get it right sometimes by good luck.

Everyone makes mistakes (idiom)

a single slip may cause everlasting sorrow (idiom)

if you say too much, you're bound to slip up at some point (idiom)

a tiny lapse can lead to a huge mistake (idiom); a minor discrepancy leading to enormous losses