What could theoretically have happened is an ephemeral chain fork, and then a chain reorganisation.
This happens when part of the network accepts a block containing your transaction to be canonical, only to find that another larger part of the network has accepted a different version of the same block to be canonical.
The version of the block containing your transaction is then marked as invalid, and your transaction put back into the transaction pool. If there are other, higher-priced transactions in the pool, then your transaction will have to wait for its place in the market.
My dapp relies on live-sync and this would be a game breaking.
I'm not sure how you're defining "live-sync", but a couple of things to think about.
Firstly, your transaction may or may not be mined into a block in what you consider a reasonable time, depending on the load on the network, and the current state of the gas price market. If your transaction is priced too low, then it is less likely to be included in a block in a timely manner.
Secondly, it's worth waiting for a certain number of confirmations before considering a the block containing your transaction as valid. (This previous answer recommends 12. I'm not sure if this number is still standard practice.)
(Here's an article on handling chain reorganisations from the perspective of Dapp development. External link.)