It appears that Google is testing showing a red squiggly line in the Google search box as you type, if they feel the spelling of your query is wrong. You know, the red line you see when you have a spelling mistake when using Microsoft Word or other word processors.
At first, I thought it was a built-in spell checking feature of the browser, but after seeing additional reports and then seeing it myself last night, it seems to me to be a new feature Google is testing.
Here is a screen shot of it in action:
I’ve had several other people confirm Google was showing them this red squiggly line for this search query last night. This morning, I am no longer seeing it.
I received a report on this from an anonymous reader, saying he is seeing it for other search queries. He showed me some brand searches, all spelled correctly, but Google showing it as a spelling mistake by showing the red squiggly line underneath it.
Again, this is not a browser spell checking feature, it was Google adding spell checking to the search box.
I asked Google to confirm this and have yet to hear back.
Postscript: Alex Hall commented, noting that Google has added a background image at google.com/images/experiments/wavy-underline.png which is the exact same color and wave of the underline we reported above. This is just one more piece of evidence that this is indeed a Google test.