Personal usage is an exception to the plain meaning rule. It allows extrinsic evidence to show the true meaning of terms the testator habitually used in an idiosyncratic manner.
E.g., if you have a friend named Jeanilee, but you exclusively and oddly call her "Vicky" in your folly, extrinsic evidence would be admitted to resolve who Vicky is.
Extrinsic evidence will be admitted to resolve latent ambiguities under both common law and modern law.
Latent ambiguities can also be partial fits. If no one person fits the description, but multiple partially fit it, extrinsic evidence will also be allowed to resolve the ambiguity.
Under the no reformation rule, courts cannot correct mistakes in wills. They will follow what the will says even if it is not what the testator wanted.
Despite still following this rule, modern courts often try to construe things as ambiguities to allow them to fix mistakes.
Then some modern courts, like California, will not follow the no reformation rule, and will just do whatever they think is the best if there is clear and convincing evidence of a mistake.
If devised stock is split between execution and death, most courts give the split stocks, not just the number of shares in the will.
Many courts also give dividends along with bequeathed stocks.
A will can be conditional, but explaining a reason for making a will does not make it conditional under the modern majority rule.