Your question is very vague and I'd need more details to give you a full answer. In short, a judge can invalidate a will for a variety of non-contested issues. For example if a gift in a will lapses because the heir/devisee died before the decedent, the judge can invalidate the will or at least that part of the will. Contact a probate attorney with more details. Feel free to give me a call if you need any help - (651) 698-2181.
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You need to consult a probate attorney in the state where the person died or where they owned land or resided. Ideally a probate attorney in the county where probate is occurring. Generally someone would need to file a motion or petition of some sort to challenge the validity of a will if on its face it was valid (i.e., signed and dated at the end)
Informal Probate is reserved for those matters which are totally (more or less) routine. If there is something unusual about the filing, such as only a photocopy, rather than an original, of the will can be found, the filing is likely to be rejected for the Informal process. Depending upon the reason for the rejection, there's normally a very good chance that the will can be probated formally. (If the legal requirements of will are not satisfied, such as an unsigned document, or a document which was signed but not witnessed by two adults, then it will most likely not be able to be probated at all.)