It does happen, I've seen clogged up hoses at work quite a few times.
You find a hard blob of blue (its always blue!) stuck up the tube, this happens when the screenwash goes back to gunk at the bottom of the tank. Most of the time the problem can be fixed with some boiling water and a minute with the pump on, but it can set like plastic in there if you don't use them often.
A few years back on SCN, one of the users with a Yellow Ibiza filled up with Halfords screenwash, used it and found the green colouring had dyed his paint. No polish could remove it either, I'm sure they had to pay up for a respray in the end!