Bitcoind live wallet backup (by copying wallet.dat)
In some cases, bitcoin-qt/bitcoind may regularly (and even gratuitously) flush the wallet [1], so it's possible that wallet.dat could be in an inconsistent state at the time you make your own copy.

The backupwallet RPC call should be a safe way to accomplish this.

[1] https://github.com/bitcoin/bitcoin/issues/10236