Many years ago I had a chemistry teacher who would tut tut disdainfully if you happened to get your valences wrong, and declare to the class at large "Trap for young players. Trap for young players!" before zoning in on one of the class nerds who invariably knew the correct answer whilst you hung your head in shame, red blushes burning your cheeks.
I was reminded quite forcefully of his sentiment yesterday when I went to insert the lining on my Liberty dress only to discover that I had sewn the lining inside out. Bugger! Trap for young players indeed.