When a stock is trading over $400 a share, you might think it would be impossible for its price to rise over 20 percent in one day — you’d be wrong.

Following yesterday’s first quarter report in which Google blew right past Wall Street and comScore’s (more on that in a bit) expectations (our coverage), the company is surging. The stock closed yesterday at $449.54, and this morning — at the time of this writing — it’s at $546.81, an increase of over 21 percent.

Unless there is a sell-off at the end of the day (perhaps tempting, given where the stock was just yesterday), this will be the largest one-day stock gain in Google’s relatively brief but impressive existence. The previous best day for the stock was all the way back in October of 2004 when the stock jumped over 15 percent, according to CNET. Back then the stock was “only” in the mid $100-a-share range. Today’s surge is simply much more impressive.

ComScore, the Internet marketing research company, is not fairing as well as Google. One of the main catalysts for Google’s rise today was comScore’s report from a few days ago that suggested the company’s paid-click growth in the U.S. would be a weak 1.8 percent (our coverage). The actual global number was 20 percent, with 10 percent in the U.S. market.

While Google’s stock was rising yesterday in after-hours trading, comScore’s was plummeting, falling 8 percent. It has bounced back today during regular trading (down only 2 percent), but there is no question that comScore’s reputation has taken a hit. People are going to be second-guessing its numbers from now on. That is probably a good thing. It can be misleading to only follow one company’s numbers and base decisions on that — something we’ve alluded to multiple times (here, here, here and here).

Silicon Alley Insider feels comScore is getting a bad rap today. They argue that it did, after all, help alert analysts to Google’s paid-click deceleration, which in turn led analysts to cut expectations and paved the way for Google to beat the Street. That’s all well and good, but the point of these reports is still to be right, no? Not simply be “directionally” correct and temper expectations. The fact is that comScore was off — significantly off.