0 fee transactions - possible that they never confirm? I've made a Bitcoin tipping service for reddit: https://news.bitcoin.com/bitcoin-tipping-reddit-minitip/

Currently users pay a flat 0.1 mBTC miner's fee to tip others since it's done entirely on-chain (which is a big deterrence from using the service). I had a thought though.

What if I made tips from user-to-user pay 0 transaction fees and make them available for withdrawal even at 0 confirmations?

The way the service works is that a user deposits say 5000 bits (5 mBTC) to a custodial address. This deposit appears in their balance after 1 confirmation.

So my thinking is, now that the amount in the balance is confirmed, the custodial account can send portions of that balance as tips to recipients who need not fear double-spends since the tips are not being sent directly from the sender's account.

The only issue I can see is that tips never get confirmed due to 0 fees, so I wanted to clarify. Is it possible that the transaction is never confirmed if the fee is 0?

Thanks
Yes. It is possible that a transaction is never confirmed, even if it does pay a fee. Transactions that pay more fee take precedence, and after a while your old unconfirmed transaction will drop out of the mempool.

Edit: This answer originally stated that older transactions get prioritized, but that behavior is no longer active on the network.