Ace etymologist Barry Popik pointed out that Woodrow Wilson once included analogous advice in a letter he wrote in 1916 [YQW]:

Never … murder a man who is committing suicide

In 1888 a variant of the quotation is attributed to Napoleon. The phrase “making a mistake” is used instead of the longer clumsier “in the process of making a mistake”. In this case, the older quote is more streamlined than the more recent variant [ABN]:

He who takes up arms against another commonly wounds himself, and the avenger should remember the words of Napoleon, “Never interfere with your enemy when he is making a mistake.”

In 1879 the quote is presented as a “maxim in war” that is unattached to any famous figure. The term “false move” is used instead of “mistake” [AW]: