What happens if a new will does not revoke a prior will?

When you write a will, the first paragraph usually revokes all prior wills. The typical language says something like "revoking all prior wills." Along with the date, this acts as a revocation of all prior wills.

Now what if a person scribbles down his or her last wishes in a hurry and leaves out this sort of language?

Pennsylvania allows for revocation by implication, which means that when two wills clash, the later will is held to revoke the earlier one. This basically means that if a will has a later date than another will does, the newer will revokes the older will.

Under the Uniform Probate Code, such a presumption of revocation is rebuttable by clear and convincing evidence. If the subsequent will "makes a complete disposition" of the estate, the inconsistencies are revocations and replace the prior will terms. If the subsequent will "does not make a complete disposition" of the estate, the inconsistencies are merely supersession and supplement to the prior will's terms. The continuing life of the prior will depends on whether inconsistencies are effective. U.P.C. §2-507(b)-(d).