Yea, they're sneaky.
I usual watch videos on youtube and if they look suspicious I turn the volume down enough to almost and whisper and scroll down off the video onto the comments, If I hear a scream or such, I press the back button.
I saved myself using thing once when there was a supposed cursing baby, turned up that the thing's mouth goes wide and screams like a demon.
I much rather get a meatspin popup then a screamer. They always scare me shitless, and it's not even totally the image, it's mainly the screaming. Actually as I'm typing I'm afraid to go into my room and turn on my tv.
I've been duped on newgrounds too. Games that sounded cool, I click on them, click the play button and up pops a holocaust/war picture, can't tell but rap music plays and there's a buncha bodies. Scary shit it is.