The Cosmos protocol states that validators are subject to punishment in case of malpractice. This is referred to as "slashing" and can occur in the wake of two events: the validator signing two blocks at the same height ("double signing") or the validator being offline for roughly half a day ("uptime"). The penalty for double signing is 5% of the total stake. The penalty for breaching "uptime" is significantly lower at roughly 0.01%. Penalties are imposed on both, validators as well as delegators.
Therefore, we advice you to carefully choose your validator. Please note, that the costs for running a secure and professional validator need to be covered by the commission rate. Hence, it is worthwhile to accept a certain amount of commission in order to secure your profits and minimize the risk of punishment.