An over-subscribed validator is one that has more than 512 nominators. A validator will only hand-out rewards to the largest 512 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 512 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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