The challenge is to think of example where it results in ambiguity about double-spending (which could either admit a double-spend or forking). Can you think of an example?
1. If I own a majority of masternodes, I have a greater than 50% chance of getting deterministically selected to confirm an instant X transaction lock
2. I submit an instant X transaction to a merchant
3. My masternodes confirm the lock on the transaction
4. Merchant accepts deposit and takes irreversible action at 0 confirmations
5. I double spend the payment in another instant X transaction which also happens to hit my masternodes
6. My masternodes dump the previous lock and report this new one as valid
7. Due to propagation delays, there is now ambiguity about which lock is valid
8. Probable fork occurs, or a double spend is achieved
Take 50% and scale it down to whatever proportion you like, the probability of this being successful is proportional to my number of masternodes.