With a weekend box office of $10.5 million, Scott Pilgrim vs. the World, which cost about $60 million to make, had the alarmingly bad opening that fans feared it would. While a few (generally older) critics slammed it, it was really a stellar movie, and one needn't have read the graphic novels it's based on (I didn't) to appreciate it.
There are plenty of theories as to why such a buzzed-about, well-marketed, solid movie performed as weakly at the box office as it did: CinemaBlend surmises that it existed in a no-man's land, with people above 30 not 'getting' it, but people too far below 30 not 'getting' the retro gaming references either; it being a geek movie that most geeks don't actually relate to ("This is a movie about a slacker musician whose biggest problem is choosing which of the two hot girls he's dating he most wants to sleep with"); and, lastly, the 'everyone hates Michael Cera' hypothesis, which I don't entirely buy, but which is widespread enough that it has to be based on some reality.
In some respects, it doesn't matter why; what matters is the aftermath. Hollywood's backlash against 'geek' properties without a hit-you-over-the-head mainstream hook could be just as bad as we fear.