I was told it's bad to use glass lids on pots that don't fit because the metal against the glass could shatter it. Is this true?
In general (wrong size lid) isn't an issue ... the glass lids, however, could be.
The main issue is when you're boiling stuff, the lid jiggles as the pressure builds up and releases. This results in the lid getting hit over and over again. At the edge of most glass lids, there's a metal ring, so the metal can help to absorb some of the hit and spread it out; also at the edge, the shape of most glass lids will help it to take the force.
With it on a smaller pot, the hit's not on the edge where it could be more easily absorbed, and you have uneven heating (the stuff outside the pot not heating up) which would cause additional stresses in the lid ... and with enough repeated strikes could cause it to fail. Depending on the type of glass it's made of, it could simply crack, or if could completely shatter if it's tempered. And shattering is just a mess -- you basically have to scrap all exposed food, for fear that some glass bits might've gotten in there.
That's not to say that a glass lid can't shatter under normal usage, but most companies test them, and wouldn't sell them if there was a high risk of breakage under normal use. I don't know how much testing goes into mis-use, but they can always claim that's not their fault, and have a possible defense against lawsuits should anyone get injured from using the lid on the wrong sized pot.
.... all of that being said, I do have a pot with a glass lid ... and I have used it on the wrong sized pan before ... but it was an oversized pan. (if you're wondering how, it was steaming stuff in a wok), so the issues aren't the same as in using it on an undersized pot.