Seeing the same recieve address on an altcoin.

[tom_o]

If for example, I noticed someone using terracoin had the same receive address as my bitcoin one, could I use my bitcoin private key on the terracoin network to spend all of that person's coins? Surely this could work as they both use the same algorithm to derive the public key from the private one.

Seems like quite a big security flaw, although ridiculously unlikely to happen, it could..

[Hazard]

if both coins share the same prefix, then yes.

such an event is almost statistically impossible. you're more likely to win the lottery 10 times in a row.

[Fallout]

Don't all coins have their own prefix in the address? Like BTC starts with 1, LTC with L and so on? Or is this prefix not considered while deriving keys?

[jackjack]

if both coins share the same prefix, then yes.

Even if not the same prefix

[tom_o]

Don't all coins have their own prefix in the address? Like BTC starts with 1, LTC with L and so on? Or is this prefix not considered while deriving keys?

if both coins share the same prefix, then yes.

Even if not the prefix

Is the prefix missed out in calculation then?

[jackjack]

Don't all coins have their own prefix in the address? Like BTC starts with 1, LTC with L and so on? Or is this prefix not considered while deriving keys?

if both coins share the same prefix, then yes.

Even if not the prefix

Is the prefix missed out in calculation then?

Yes
Where you send the coins isn't the address but the hash160 of the public key (ie step 3: 010966776006953D5567439E5E39F86A0D273BEE in the wiki example)
The prefix is only added in step 4

See https://bitcointalk.org/index.php?topic=220946.msg2343066#msg2343066