The Classic Environment (or usually just "Classic") is just OS 9 plus a little extra software to allow it to run under OS X. If something makes Classic crash, all the applications in it may crash, but all your OS X applications should be fine. Many of the usual OS 9 functions (e.g. networking) are handled by OS X, so you can trim down the ExTensions and ControlPanels extensively.

ClassicSpy puts a menu on the right side of your menu bar that shows -- and lets you switch -- Classic on or off.

Q. I upgraded to 10.1.3 and now Classic Environment crashes.

A. Are you booting Mac OS X from a UFS volume? Don't do that. Although officially supported supported, 10.1.3 on UFS apparently breaks in horrible ways, including breaking Classic altogether. Stick with HFS+.

Q. When I start up ClassicEnvironment, it gives me this error message:

You do not have permissions to run Classic from /System/Library/CoreServices/. 
Fix permissions and re-run Classic. 

A. This can happen when you install OS 9 after installing OS X, or if you upgrade OS 9 from 9.1 to 9.2 without ever having run Classic through OS X. The problem is that ClassicEnvironment does not think that your OS 9 system folder is blessed (even though you can reboot into OS 9). To fix this:

1. Go to SystemPreferences, StartupDisk pane
2. Select your OS 9 system folder
3. 'Do not restart'
4. Quit SystemPreferences. It will ask if you really want to change the startup disk, click Save.
5. Start ClassicEnvironment by running /System/Library/CoreServices/Classic.app
6. It should prompt you to copy files into your OS 9 system folder, click OK.
7. Go to SystemPreferences, StartupDisk pane
8. Reselect your OS X system folder, and quit. 'You do not need to restart.'

A. Apple now has a knowledge base article on this: KBase Document 106678: Mac OS X: Classic Will Not Start Up - Mac OS 9 Not Installed or Not Blessed

Q. I dont like the idea of classic wasting my disk-space. I am a new mac user and will never need to run classic applications. Is there a safe way to delete it?

A. The short answer is sure, delete away. You'll want to delete the folder called "System Folder" (NOT the "System" folder), and the "Applications (Mac OS 9)" folder. That's really about it.

Note that you won't save tons of space - about 300mb for the system plus whatever classic applications you have. Also, if you've worked at all while booted in OS 9, check application folders for documents you might want to save. In particular (i believe), iTunes under 9 by default saved the music library to the application folder.

Q. When I print multiple jobs to different printers from a Classic app, all of the jobs seem to spool through the old Print Monitor application, and in a serial manner (ie, even jobs to different printers wait until the first job is through). Is there any way to spool/print jobs in parallel, ala desktop printers?