That's called the "Money Shift" for obvious reasons. This is a mechanical over-rev, the engine RPM's are determined by your speed and gear selection, not the other way around. The rev-limiter, then, will not help. If you are near redline and drop a gear down, the engine spins too fast for its design. The most common damage would be valve impact caused by valve float beyond the design limits. Throwing a rod or a variety of other problems could occur as well. None are usually good. The wheel spin may have helped you actually, by limiting the rotational speed you were transmitting from your drivetrain back through to the engine. I hope I am making sense here, or maybe you already get this, the wheels were turning slower than they were when you downshifted, spun (relatively) backwards, and induced your near spin. That's a lot of torque change and, hopefully, all you'll have is a broken mount. You need to get it checked out. I would NOT drive this car until you do.
Best of luck!