help, this is spooky and annoying

three times so far, when working in ps7, a mysterious red line - like a thread - has appeared on all layers of the picture i am working on. it usually goes from the left top corner to the middle right.

i cannot get rid of it, and i do not know where it is coming from. it is not straight, and it is about 1-2 pixels wide. what kind of bug do i have there?!

*too tired to redo the whole pic*