Okay, I can't believe I actually bothered to look this up, but the California law regarding helium-filled mylar balloons is found in the state penal code, part 1, title 15, chapter 2:
653.1.
(a) No person shall sell or distribute any balloon that is constructed of electrically conductive material, and filled with a gas lighter than air without:
(1) Affixing an object of sufficient weight to the balloon or its appurtenance to counter the lift capability of the balloon.
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(2) Affixing a statement on the balloon, or ensuring that a statement is so affixed, that warns the consumer about the risk if the balloon comes in contact with electrical power lines.
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(3) A printed identification of the manufacturer of the balloon.
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(b) No person shall sell or distribute any balloon filled with a gas lighter than air that is attached to an electrically conductive string, tether, streamer, or other electrically conductive appurtenance.
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(c) No person shall sell or distribute any balloon that is constructed of electrically conductive material and filled with a gas lighter than air and that is attached to another balloon constructed of electrically conductive material and filled with a gas lighter than air.
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(d) No person or group shall release, outdoors, balloons made of electrically conductive material and filled with a gas lighter than air, as part of a public or civic event, promotional activity, or product advertisement.
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(e) Any person who violates subdivision (a), (b), (c), or (d) shall be guilty of an infraction punishable by a fine not exceeding one hundred dollars ($100). Any person who violates subdivision (a), (b), (c), or (d) who has been previously convicted twice of violating subdivision (a), (b), (c), or (d) shall be guilty of a misdemeanor.
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(f) This section shall not apply to manned hot air balloons, or to balloons used in governmental or scientific research projects.
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(Amended by Stats. 2004, Ch. 193, Sec. 141. Effective January 1, 2005.)
Note the date, though -- this exact wording of the law wasn't in effect during the run of SOTC, and I definitely don't feel like trying to find old versions of the statutes.