so .. i guess it appears as an orphaned transaction on the "wrong" chain ?
could be useful, not sure about how harmful it can be for the "other" chain though..
No it doesn't appear as an orphan. Basically what happens is this: Lets say you want to donate 10 BTC to me (yes please!) but without noticing it, your using your Terracoin client and you send me 10TRC instead (to the same address).
Nothin happens on the Bitcoin network- the network is entirely unaware of what you intended to do. You send TRC from the TRC client to a TRC address. I have the private key of both the TRC AND BTC address. So there are no orphaned transactions, your transaction always happens only within it's own chain, and to everyone watching, just looks like a normal transaction.
Remember, the coins you send will always go to the chain that you are sending them from. BTC and TRC will never intersect. They are totally different networks. The only thing is that the private key for a BTC or a TRC address will always be the same, you can have one "address" that receive BTC or TRC. But compared to USD they won't have the same value.
IE: I prefer getting 10BTC not 10TRC. :-)