Whenever
matrushkaka or I screw up I'll shake my fist dramatically and shout "Let this be a lesson to you!" I'm not attempting to call anyone an idiot when I say this - it's just that failure is eduational. There are valuable lessons in failure, even epic failure, and if you learn from your failures you won't do them again. Conversely, if you don't learn from your failures or insist that bad luck rather than insufficient preparation and risk mitigation are to blame you will end up just making the same mistake over again. The dramatic, fist-shaking delivery is for comedic and dramatic effect. "Let This Be A Lesson To Me" (LTBALTM) and "Let This Be A Lesson To Him" (LTBALTH) are similar phrases.
Example: today I cross-threaded the plastic pipe on the base of the
fill valve on my
new Toto UltraMax while attempting to install the Washlet. I am back to using 19th century paper technology and while I wait for a replacement fill valve. LTBALTM.