Tested again and no error. Here is what I did.

Create:

200GB Target (Write Back enabled)
20GB Snapshot (defaults) Set my Snapshot schedule to run at xyz - no end time (also manually)

Copied data on the target
Then Snapshot kicked off (verified data on the snapshot - good)

Then stopped the task - no error 21 only error 57 (I was sending data and tested again with stopping the task).
Error57:
"The system failed to flush data to the transaction log. Corruption may occur."

Then Deleted the Snapshot (in ms disk mgr.)- then stopped task then re-enable the Snapshot again then Deleted the Target as well (just to push it) . No Error .

Also tested with Deleted the Target while Snapshot was scheduled and still no errors.

I only tested with XP. I can provide a web demonstration showing this if anyone is interested.