Also one Troll Lord book sort of broke the rules. The authors inadvertantly used a few things out the official D&D spatbooks (which weren't OGL). I don't think WOTC did anything, I think Troll Lord caught the mistake themselves and issued an update and squared themselves with WOTC.
If WOTC wanted to enforce the OGL, they'd be pretty busy, because even to this day, some companies still forget to include the SRD in section 15. Or their own product.
Like for instance, the World's Largest Dungeon. Given it's such an expensive book, I was suprised to see that they didn't include an entry for itself. And even though the book is based on the revised srd (ie, 3.5), they cited the original instead.
Or Spycraft. It includes the Star Wars RPG in section 15 of the OGL.
And a couple companies I know have used stuff from the modern SRD, but either cited the original instead, or just didn't cite it, period.