Black "terminal" screen on startup

When I power up my iMac, I'm getting a black screen with white lettering that scrolls very fast, then stops, & won't go any farther until I type "exit" (then hit return) -- and about half the time, it doesn't respond to that, either. If it does respond to the exit, it goes on to the normal password screen. If it doesn't, it just sits there and remains at this black & white wall of text. It looks like the old PC Windows startup process.

The times it just sits there, I hit the power button and do a hard-restart, and the Mac will go to my regular password screen.

This isn't terminal, or at least it doesn't look like the terminal application. Terminal is NOT in my startup items; I've already searched the forum and tried that as a fix, and it's not there. This started two days ago, when I upgraded to Mountain Lion, and it's got me dead set against upgrading any of the other Macs to ML. Okay. What's going on?