A will may be found invalid because something is physically wrong with the document, or the way it was executed. A will is required to be executed in a very specific way.
A will is required under Florida law to be signed by the testator at the end, and signed by two witnesses who observed the testator sign it. A will with no witnesses, or one witness is invalid.
A will where the witnesses did not actually see the testator signing, but added their signatures later, is invalid.
A will with pages missing is invalid. A will which appears to have pages removed and different pages inserted is invalid. The testator’s signature might not look genuine, and could be challenged as a forgery.