A Guaranteed Way to Corrupt SD Card
If the data is really important, you should use a modern checksumming CoW (copy-on-write) file system like btrfs or ZFS. A recent kernel+btrfs can do "raid" on a single device, storing the data twice with checksumming to determine if one block is damaged, and repair from the (hopefully correct) copy if it happens. Copy-on-write guarantees that data is never overwritten and any eventual changeover happens atomically. Combined with features like snapshots for reliable factory reset, and automatic compression makes it a good choice for space-constrained storage in embedded systems. The main drawback is that it hasn't seen nearly as much testing as "less reliable" file systems, which can be a huge turn-off.

To answer your first question about data corruption, you need to add more details. It is unclear what you expect to happen. The failure mode for Flash storage is typically that you can no longer change a bit from 1 to 0; they are permanently 1. You also haven't told us which file system you are currently using, so I can't advice on sectors and places where you could write random data. Liberal sprinkling of 0xFF to random bytes in the card should do it, perhaps even 0xFF..FF on whole sectors.