An over-subscribed validator is one that has more than 256 nominators. A validator will only hand-out rewards to the largest 256 nominators. Therefore, when voting a validator that ended up being over-subscribed, you run the chance that you will not receive rewards if your staked nomination is less than the largest 256 nominators'. Usually, the more over-subscribed the pool is, the greater the chance that you will not receive rewards.
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It is difficult to know how many nominators will nominate a validator in the next era, however one can rely on the current era's subscription value in order to make an estimate. For example if validator A has 850 nominators and validator B has 50, it is more likely that the former will be over-subscribed in the next era compared to the later.
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For more information on staking Kusama (KSM) with P2P.org, visit https://p2p.org/kusama.

For additional staking support, visit the P2P Kusama Help Centre.
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