A common mistake is not having the daughter card inserted correctly, either hanging over the socket by one pin, or in backwards. In this case you will usually just see the powerup garbage on the screen. This garbage is just the random bits that are in the character RAM at powerup, and it indicates the CPU was not able to run.
Another common mistake is for the Z80 CPU to have been inserted incorrectly, either backwards, or possibly with a pin bent that did not insert into the socket properly.
The Mario Bros high score save kit will perform a very simple powerup selftest of the HS daughter cards on powerup. Should this mini selftest fail, the text "HS FAILURE" will appear on the bottom line during the very first attract mode screen. After that, it is cleared by the subsequent attract mode screens, so you need to check for this immediately after powering up the game. A sample screen showing the failure is shown to the right. If you see this failure, most likely that means that one of the socketed chips on the daughter card is probably lose. Check to make sure the 8 pin dip (the smallest chip) on the HS daughter card is fully inserted into its socket.
Mario Bros. High score failure