It sounds like you are being front run. This means that a bot is watching you send a transaction, reading the input (specifically, _answer), and submitting a transaction with the correct answer but a higher gasPrice. A miner will accept their transaction before yours, causing theirs to succeed and yours to fail.
You can see that this is, in fact, what is ...
People have been explaining this since Bitcoin emerged on the scene but it's a little difficult to grasp at first and usually takes a few explanations from a few angles, so I'll try for an overview that zeros in on your specific question.
Bear with me as a layer a few concepts.
Blockchain consensus is mainly concerned with reaching an agreement about the ...
When the caller is an smart contract if it can re-enter BuggyWithdraw and can change balance[msg.sender] then it will be possible that BalanceDecreased is never emitted.
So an off-chain service that relies on that event might not take into account the balance has decreased.