What are some things to know if a console is about to fail??

[misterd]

Might be easier if we had a list of failed consoles for comparsion.

I'd define a "failed" console as one that doesn't survive to the end (or near the end) of the generation it is released in, especially if it doesn't have a follow up system.
Neo Geo

Definite fails:

Neo GeoSega Dreamcast
Atari JaguarPanasonic 3DO
Sega Saturn 

Note1: I am not going back to the pre-NES days. Market has changed too much, IMO, for good comparisons.

Note 2: Most of these systems were also launched with much higher prices than the competition. The exceptions are the DC and the Jaguar.