What can be the consequence of being caught subletting an apartment w/out the leasing office's approval?

Castration.

You may be in violation of the original lease and subject to litigation and civil penalties.

Eviction.

Well, the leasing office can file for eviction to evict the people living there and you. Then they can make you pay all the lease payments to them for the length of the lease and if you don't pay they can report it to the credit bureaus and get a judgment against you. You will need to read the lease agreement in full. If the lease agreement doesn't say anything about sub leases then check the laws in the state you live in and if there are no laws against it then it would be completely legal and the leasing office could do absolutely nothing about it.

You can be held responsible for any damages caused by the new tenant and for any unpaid rent.
That's about it.

It will go on your permanent record mister, so behave yourself! :)

It depends.

If your lease does not prohibit subletting or if you have no lease at all, they can't do anything. They can try, but they won't win.

If your lease does prohibit subletting, you will be served with a notice which will give you the opportunity to correct the situation. If you don't, they can file to evict.

What does you lease say? I'm sure it's right in there, what the consequences will be for subletting. Read it, you signed it afterall, you should probably know what it said.