Polyself bug

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The polyself bug was a fairly serious flaw in the handling of self-polymorph in the case where the character polymorphs into their own race ("You feel like a new <race>!"). It is listed as SC343-10:

SC343-10: Player can raise HP and Pw by dancing with vampires.

To exploit this bug, a player would need to boost their hit point and energy maximums through any applicable method, e.g. drain for gain or simply drinking enough non-cursed potions of healing, extra healing, full healing, and/or gain energy. The player then level drains themselves to reach experience level one - methods to level drain yourself include the drain life spell, hitting yourself with Stormbringer or The Staff of Aesculapius, or the attack of a level-draining monster, e.g. any wraith or vampire, as alluded to in the official description of the bug.

Once this is done, the act of polyself into your own race has a chance of adjusting your experience level. A polyself into your own race at level one is highly likely to kill you from dropping it to 0, but can also increase it to 2 or 3 - increasing your level by polyself will approximately double your hit points and triple your power, and draining your level back to one does not fully offset this.[1]

This means that, with several amulets of life saving, repeatedly polymorphing into your own race and draining level back to one can significantly increase your hit points and power, usually to the cap of a 32-bit signed integer. Pudding farming is often used to acquire a healthy quantity of amulets for this purpose.

This bug was fixed in Nethack 3.6.0.