I'll have one of those Guiness shandies where the Black Stuff is floated on top of the White Stuff - or is it thy the way tound - anyway, if he mixes the two, it all explodes all over the bar and the barman has to give you another one...

These fellas think that just because they know how to make a shamrock in the froth on a Guinness they know all about pouring it right.