An idea I mentioned to somebody else that I don't want to forget
Prohibition: A few things work out a bit earlier than expected, and by 1905 there are working electronic computers. WWI begins earlier (everybody thinks computers will give them the edge), and is equally destructive, but it also ends earlier. By 1917 Edison Electronics has made the New York-Berlin-London market the fastest, biggest stock market in history. In 1918 the Blitzkrieg Crash takes almost everybody by surprise (not Poincare, whose work on chaotic dynamics guided the investment strategies that made him one of the richest men in America after the crash). Unemployment soars; Edison Electronics factories making prototypes of "office computers" are torched down by angry mobs.
The Computer Prohibition Law passes: no (civilian) computers are allowed in the US.
But people of all sorts have tasted the forbidden fruit, from greedy businessmen to dreaming scientists. The demand exists, and when there's a consumer, the market finds a way.
The year is 1921, the place is Chicago, and if you need something smart, Al Capone is your guy.