If the clutch is brand new, you can warp the Pressure plate/Flywheel, or tear up the clutch DIsk.

The reason for a "break in" period on a new clutch is to give it time to "mesh" together with everything. Certain fibers and composited of a clutch need to be worn in to each other before you can go off an beat on it.

There are certain clutches that come with kevlar disks, that rewuire zero break in, the material is so hard, and so abrasive, it doesnt need to be broken in. theoretically, Kevalr should never wear out either.

but to answer your vague question:
1) Used engine that has not been rebuilt, drive it like you stole it
2) New Clutch, take it easy for 500 miles to make sure its ok

If you want to know more, read this thread, it is one of the best i have ever read, and it has lots of good info.

Clutch 101 By Rocket Motorsports