This is two replies on the National Electric Code Internet Connection forum that I referred to earlier.

From: Stauffer
Dear Mike: UL tests and rates circuit breakers by one manufacture (such as Murray) for use in other manufacturers' panelboards (such as Square D). I believe this is called "classification."
The bigger, more reputable manufacturers hate this practice and always have, but UL does it nevertheless. Therefore, I believe the panelboard manufacturer's warning label is merely a scare tactic, and that anyone can install a properly classified breaker in another brand of panel.
Of course, there probably are breakers that are mechanically compatible with other brands of panel but that have not been classified for use in them. (Classification, like listing, costs money.) This is a grayer area for which I have no good, short answer. I guess a careful, liability-minded electrician or contractor would check his UL listing directories before installing a Brand "A" breaker in a Brand "B" panel.
BROOKE STAUFFER, Director, NECA Codes and Standards
From: pauleyj@squared.com
Mike: Seems you had a wide variety of comments. Most seemed to indicate that the practice of installing something that the panelboard manufacturers label clearly prohibits would be putting yourself way out on the limb.
Square D's position is simple. Our panelboards are clearly marked with what circuit breakers are permitted to be installed. Installation of anything contrary to that marking will void our warranty and we will certainly make that point loud and clear should something go wrong with the installation. This is not only based on our listing, but also takes into account the 40+ years of design and application expertise our engineers put into the circuit breaker/panelboard combination.
I have seen many comments that indicate that this is only a "scare tactic,” etc. However, keep in mind that the product standards have, for many, many years, required that the panelboard be marked with the circuit breakers that are permitted to be installed, and for good reason. When manufacturer “A” says that their breaker can be installed in manufacturer “B” catalog XX-XXXX panelboard, I wonder which version of that panel they were talking about. The version made in 1998, 1993, 1989, 1985, 1980, etc? Keep in mind that the catalog number might be the same, but the design changed many times. This is where the panelboard manufacturer spends a great deal of time, making sure that things are compatible. Manufacturer “A” would have no idea what the "history" of that product was through all those years.
The vast majority of electrical inspectors (and electrical contractors) will tell you that they go by what is marked on the panel. Do anything else, has too many variables.
Jim Pauley, P.E.
Square D Company

I've also found some googled replies on various forums from over the years where a person is stating their electrician pointed out wrong breakers in a panel. The homeowner then states that they had the home inspected two years ago when they bought it and that was not called out by the inspector. The electrician then replied that most home inspectors don't know what to look for and he should have had the house inspected by licensed trades contractors who are more knowledgeable!