Chernobyl wasn't caused by any small error or minor oversight, it was an idiotic design put through an idiotic "test" where they basically did everything they could to make it blow up.
Some epic failures I can think of that were caused by simple/small problems:
DeHavilland Comet - was the first jet airliner, and many of them blew up in midair due to tiny cracks at the corners of the square windows. That's why modern airliners have rounded windows.
V-22 Osprey - most of the earlier crashes in its crash-filled history are due to flex joints in the hydraulic lines that allow the wings to fold. The wings have to fold so that it takes up the same space on a carrier as a helicopter. Turns out the flex joints weren't so good at flexing.
OpenSSH keyspace vulnerability - because somebody left a piece of testing code in place that was clearly marked as testing code. One small block of code, every Linux computer on the planet had to regenerate keys.
A website I maintain once went titsup because of a stray semicolon on an IF statement - that made it always return true, but it kept running as long as it got new entries in this event planning area. One day it ran out, and this line of code that was common to all pages got stuck in an infinite loop, trying to fetch new events when none existed. One semicolon, whole website down. Stuff like this probably happens a lot in software.