I subscribe to a Google Service called Google Alerts. Anytime certain keywords that I want to watch for are indexed by Google I get an email from them telling me about the new pages on the web that have those keywords.
The email address that these alerts get sent to is my Gmail email address which has the spam filter switch on. It's the best spam filter I've seen and so I trust it but I often glance over the spam list before I "delete all" to make sure that it's doing a good job.
Today I saw a Google Alert in the spam box. The email came from email@example.com and I checked the IP it came from and it wasn't spoofed. It also contained the regular alert content.
What amazes me is that Google's own email is getting put in the spam bucket by it's own filters. At the very least I would have thought that they would have white listed their own IP. Apparently not.