The phrase 'the penny drops' describes that instant when all of a sudden, what was a complete mystery becomes completely obvious. While thinking about this recently, it struck me that there is a similar, yet opposite case, which occurs when one sees a totally different meaning in something taken as understood. For reasons that I suspect are chemical, the sense of dawning realization is identical for both.
Take the phrase 'united we stand, divided we fall' (apparently the motto of Kentucky, amongst other things). For a long time I knew this meant 'when things are good, we stick together, but don't expect any help when things get bad'. Yes, indeed, I was probably the only person on the planet to misinterpret that one, but it still seems like a reasonable meaning to me.
The same goes for the line 'The Wombles of Wimbledon Common are we!' from Wombling Along. For at least 10 years, I was convinced there was a line break right there in this song about plebian Wombles. As a side note, I was absolutely delighted to discover Google has a directory entry for this, granting the respect it deserves.