Yes, that’s dead right, you should only fill to the bottom of the filler neck.
In a gravity fed fuel system (on a bike with carburetors), there’s a gas tank “breather” hole at the top of the gas tank near the gas cap. The breather hole is connected to a breather tube which goes down to the bottom of the engine area. The purpose of the breather is to let air into the gas tank as the gas level drops through use. If it didn’t, a vacuum would be created in the gas tank and the gas would no longer gravity feed into the carburetor(s).
These breather tubes used to just vent to the open air, however, since clean air regulations came into effect, the “open-end” now plugs into a charcoal cannister, which is supposed to catch any gas which is in the *fumes* coming out of the breather tube. I.e. it allows air into the breather from outside, but filters any air drifting out of the gas tank when the bike is not running.
If you over-fill your gas tank (bike on side-stand), when you straighten the bike up to ride away, you could slosh gas down the breather tube, thus soaking the charcoal cannister. Once soaked, it cannot pass air and therefore a vacuum will form in the gas tank. After a short while, you’ll experience the symptoms of running out of gas.
If this happens, you’ll be able to tell, as when you open the gas cap, you’ll hear the air being sucked-in (and the bike would then run for a short while until a vacuum formed again).
One thing you could do if it happens to you is to pull the breather tube off of the charcoal cannister (probably under/behind the engine) and leave it off for a while to dry out; you should immediately be able to re-start the bike. Be absolutely sure it’s the breather tube and charcoal cannister you’re disconnecting! Once dry, remember to put the tube back on again.
DO *NOT* DISCONNECT THE *GAS* LINE FROM THE GAS TANK, PETCOCK OR CARBURETOR.
Alternatively, I guess you could ride with the gas cap a little loose to let air in (not recommended).
It happened to me a few times before it was explained to me what was happening, and the first time, I got the bike trailered home ($150+) as I didn’t know what was going on except that the bike wouldn’t re-start.
Hopefully, having read this, this won’t happen to you…
Disclaimer: This is given as advice only. You are responsible for ensuring your own safety.