I think that the two most common questions I get on a given day are one (or both) of the following:
1. Can you write me a script to _______?
2. What does msiexec error ____ mean?
Actually, number 3 would be "can you look at my script and tell me why it's not working?" which often takes over the number 2 spot. As for msiexec error codes, the holy grail is this...
As I would often quote from Fast Times at Ridgemont High during my construction days: "Learn it. Know it. Live it." or something close to that. The most common errors are 1602, 1603, 1619, and every now and then 1624, but I'm sure you can encounter any one of them at some point. By the way, 1602 is interesting because it can happen when no one is logged on. A common source for that is an overzealous anti-virus application. 1619 is usually due to fat-fingering the MSI filename in whatever place is trying to launch it (SCCM package definition, Altiris, Prism (eewe!!), script or GPO), or it can be permissions.
There. That's your Monday MSIexec tip of the, uhhhh, hour? Enjoy!