Slashing punishes for actions that are very difficult to do accidentally, and it’s very likely a sign of malicious intent. It’s a really rare event: only 5 slashed validators within the whole network for the last month (or 0.001%).
https://beaconcha.in/validators/slashingsWhat is “slashable” behavior? In a nutshell, it’s a violation of consensus rules in network. Now it’s a three conditions: proposal two conflicting blocks at the same time, double vote attestation and surround attestaton. It could be either a malicious specific action or misconfiguration (the most often it’s running two the same validators in network).
Slashing results in burning 1,0 ETH at once, and removing the validator from the network forever, which takes 36 days. During this time, the validator continues to work but can no longer participate in validation and block creation, getting a penalty of around 0.1 ETH in total.
Mostly, that’s all losses, but there is also an additional midpoint (Day 18) penalty that scales with the number of slashed validators, it’s called "correlation penalty”, which is currently theoretical, i.e., never encountered in mainnet, but we should understand it. This mechanism protects the network from large attacks. The math for calculation penalty is pretty complicated, but the summary is if there are only 1, 100, or even 1000 slashed validators within 36 days - it’s okay and penalty equals zero ETH. However, if the number of slashed validators increases to roughly 1.1% of all validators (currently 5.1k), this penalty becomes 1,0 ETH. And +1,0 ETH for every +1.1% slashed validators. So if 1/3 network is slashed, the penalty will nullify the whole stake (32 ETH).