Please name at least 5 of them.
Also, it's trendy to drop unseasoned developers onto it and expect them to build complex applications that are publicly accessible yet 100% foolproof. Go figure.
If you asked me, I couldn't even tell. I've encoundered some in the past, sure, but found (in docs) both rationale and the correct way to use stuff.
There was that reference-vs-value matter when passing objects around, in PHP v4.x, but that's fixed by v5.