So, I was working on a big project....
When all of a sudden, an urge to place these blocks came into mind:
(i'm pretty sure i had the width and height set to current but whatever)
When I clicked the blocks... 
freezed. That's literally it.
Note this bug likely only applies to the Turtle sprites.
Yup, that shouldn't freeze Snap!, I agree.
Jens has this strange idea that the turtle costume isn't a costume, so I wouldn't be surprised if PIXELS OF it returned an empty list or something, but freezing is a little too severe of an error message. :~)
It returns nothing (an empty value, not a list).
For me it freezes, not returning anything, just running forever, unstoppable.
Oh, it used to return an empty value, but after testing, it also freezes for me.
fixed! Thanks for reporting.
(get it? reporting? (bad pun))