It seems there is something of a standard test string for anti virus (wikipedia has more on this).
The idea is that systems that look for viruses will have this string loaded as a signature of a valid virus, and so react as such. This allows you to test virus checking systems without an actual virus being used. Obviously some systems may flag as "test virus" or some such, and some may not have this "standard" string.
The string is :-
So far, so good, but what people are doing (see tweet) is putting that in a QR code, e.g. this (feel free to copy this image).
[note the white space around the image is part of the QR code spec]
And then sticking it on a car, or a hoody, etc..
The result is that some systems that happen to log the content of QR codes they see, e.g. on CCTV and the like, promptly trip their virus detection systems. Ooops.
Of course this does raise questions of whether this could count as Computer Misuse, but then should such systems be reading QR codes off a hoody anyway?
P.S. My QR code generator is on GitHub if you want... It seems to be more efficient than most (though no advantage for this particular case), and has a lot of options (png, svg, text, binary, eps, ps, hex, data URL). Have fun.