My browser / operating system: Windows NT 10.0, Firefox 44.0, Flash 20.0 (release 0)
Ok, so this has been happening for a long time, but anyway, sometimes I import a picture into a Scratch sprite/backdrop and it turns pink. I use vector mode. This is the oddest and probably most frustrating glitch I've had so far.
Need proof? Look at the thumbnail of my featured project. It's supposed to be a picture but it's a pink rectangle.
If anyone knows what the heck is going on with that please answer or tell the Scratch Team so they can fix it.

It is a common problem in vector that is normally temporary. Reset the sprites and backdrop to the thumbnail and save again.

it happens. I don't get it anymore, but I used to. the cure? put it into bitmap. that always worked for me.

when I receive [ AAAAAA A GLITCH]
bitmap it!