Problem
There are a few ways that cause the game to unfairly and irrevocably crash. The game doesn't do an emergency save, so all of the player's progress is lost forever. Those are:
* Giving monsters poisoned meat crashes the game if they accept it (e.g. cave tigers).
* Reading a scroll of repair without carrying absolutely any other item crashes the game ("Error: No items to choose anything from."), unlike scrolls of uncursing and identify which turn to dust.
* It's said that some uniques (e.g. Gaab'Baay) stepping on a trap will crash the game.

How to reproduce it
Giving monsters poisoned meat
1. obtain a piece of raw meat,
2. dip it into a potion of poison,
3. give it to a monster that will not ignore it, such as the aforementioned cave tiger,
4. the game will crash.

Scroll of repair
1. unequip everything that appears in the "personal equipment" screen, and drop all items, except for a scroll of repair,
2. attempt to 'r'ead the scroll,
3. the game will crash.

Uniques stepping on a trap
I wasn't able to reproduce this bug.

Notes
There's a HoF final log file that confirms the Gaab'Baay trap crash:
"I had to restore from backup twice while luring her due to a fatal crash that occurs whenever she steps into a trap. This is solved by disarming all traps (especially in DF)." -- http://adom.brinkster.net/hof/flg_di...ukawa-edit.txt