From the Boston Municipal Code, 16-12.14:

No person shall in or about any part of the streets, parks, public grounds, public buildings, or other public places discharge without permission of every person who would be struck, or the owners of property which would be struck, as the case may be, as result of such discharge any toy, amusement or novelty product fitted to propel, by compressed air or gas, any substance primarily intended for amusement or decoration, in the nature of “liquid string,” so-called.

In other words, we have a law against shooting people with Silly String in public unless they’ve given express permission in advance.

I’d like to point out two important loopholes.  First, on private property I can shoot you with Silly String whether you permit me or not.  The law applies necessarily only to public property.  Second, I can also shoot you with some other substance as long as I’m confident no court would consider it amusing or decorative.

Some others that I enjoy:

1. (16-19.4) No person shall, in the Common, Public Garden, or other public grounds of the City, annoy another person….
2. (16-10.4) No person shall, within any market limits, play any game….
3. (16-12.15) No person shall, in any street, play ball, throw stones, snow balls, or other missiles, or shoot with or use a bow and arrow or sling.

I want that first one on a tee shirt.