Oh yeah, when you're using "dd" or "dd_rescue" be very careful with your parameters.

They don't call dd "disk destroyer" for nothing!

From wikipedia: It is jokingly said to stand for "data destroyer" or "delete data", since, being used for low-level operations on hard disks, a small mistake, such as reversing the if and of parameters, may accidentally render the entire disk unusable

The best way to "practice" with dd is with a system with non-critical data on it.