I found it was easy to find weird passwords for Clash at Demonhead and Metroid 1 (NES).
For Metroid, I often entered the password screen, chosen some letters and numbers, got some blank spaces sometimes, and when I was lucky, it worked! I often got games that were impossible to finish, because I found myself in places where I didn't have the items to leave and go everywhere like it should be... I also found a new ice beam. It is just as big as the wave beam, but it shoots like the normal ice beam with the blue form of the wave beams. It is not stronger, but it is more beautiful as a beam!!!
For Clash at Demonhead, I did the same thing, but it was more difficult, because we only have two chances to input a password each time we enter the password screen... Some passwords will allow us to fight Bopper at the very end, but past him, the professor Plum won't be there or sometimes, the barrier holding him won't even open... Sometimes, the Alien past the last boss won't be there neither. Lots of things can be weird! I was even able to get 2 more energy points (HP) over the normal maximum in a game!
Here is some of the passwords I found for those two games:
Clash at Demonhead: In this one, try the route 99. It is funny! :P
U5MND 3WTWF GRPJH
56PQR QRTPQ VU1VT
Metroid: Here is the shortest password I found for this game! It is impossible go farther with this one. Just see for yourself! It is really easy to understand what is the matter!!! ;)
QHIJ7M L98KJ
Metroid: In this impossible password, I have the big Ice Beam I told you about. However, we cannot finish this game.
(0 = zero)
M26jWw y77z02
027i5s 9khG?3
Have a good time!!!