This morning there was a problem with the implementation of Google's malware filters. Gmail's spam engine uses those filters (among hundreds of other signals) to help protect our users from malware, and so between 6:00 a.m. PST and 8:00 a.m. PST, we mistakenly sent some legitimate mail to people's spam folders.It's those "hundreds of other signals" that probably prevented all email from being sent to the Spam folder. The problem, you may recall, was that Google entered a wildcard into their malware filter file, thus resulting in the whole Internet being flagged.
Humorous, but possibly disastrous. Fortunately, it wasn't.
While it's great Google managed to develop an automated way to restore your email, meaning that you probably didn't miss any legitimate messages, you should still check your Spam folder manually, just in case, according to Google.