Tom mentioned debasing, and some of the techniques taken to prevent it.
He mentioned coin scrapping, but there was also official debasing. This was done on an immense scale in the late roman empire: soldiers would be promised a pay in denarii, which were a silver coin, but the silver content was re-defined when time came to actually hand out the cash. This was harder to do in the middle ages because there was not much contract law to go about and people indeed weighed gold, while in the roman empire it was possible for citizens to sign long term enforceable contracts labelled in some currency.
(Amusing irrelevant fact: the denarius' name is a derivation from the root latin word den which means ten, because a denarius was worth ten asses. Yes, asses. Make of it what you will.)
I'm not so sure about the Byzantium story. Because of currency was based on precious metals part of the whole advantage was they could increase the circulation of currency, without devaluing the currency. The value of currency wasn't based on some abstract value of the economy that produced it, but based solely on the price of the precious gold. Conversely this meant that the ruling bodies often tightly controlled the mines and industry for the appropriate precious metal, in order to have some control over the supply and thus maintain the value. Obviously if the value of the metal dropped or rose, that affected the currency, but the amount of coins in circulation should not be as important as the relative scarcity of the metal itself.
That is true, of course, but the morale of the story is that you can never control anything that happens to your metals. Since the metal production, melting and coinage has nothing whatsoever to do with actual economic activity, you get two independent variables where you only really need one.
Also, the value of money is not imposed from above through some abstract calculations. There is an actual offer and demand going on; money is worth what you will give up for it. I give 41 hours a week for a certain of money, that's what it's worth to me, not grams of gold. Gold could be a means of exchange, or many other things, but it only counts as a means of exchange, not as gold.
I believe the gold standard you refer to, the modern gold standard was a Gold Exchange Standard or a Gold Bullion Standard, where the gold standard of medieval times was a gold specie standard.
True.