The obvious thought may be "the ColecoVision, Vectrex, 5200 etc" but it's been established, at least by many gamers, that those consoles were part of an unacknowledged "3rd generation" and that NES is really 4th gen, SNES 5th gen, PS1 6th gen, PS2 7th gen, Xbox 360 8th gen, and we are currently in the 9th gen. An alternative would be to keep the Wiki generation categories but call the Colecovision/Vectrex/5200 etc "2.5 gen".
This is not about those consoles. There are some consoles that the generation is ambiguous. Keep in mind that when I talk about generations, the ColecoVision is 3rd gen and generations from the NES forward are shifted forward one generation each (i.e. the current generation is the 9th, not 8th)
The NES has been cited as an example. I've seen a thread on another forum where someone claimed the NES was in the same generation as the 5200, ColecoVision, and Vectrex. But to me, the NES is pretty clear cut as a 4th gen console. Yes, it DID launch as the Famicom in Japan on July 15, 1983, which was only a year after the first 3rd gen consoles first launched. But the NES remained a "current" system through the end of the 1980s and lasted well into the 1990s, getting its most software in the 1988-1991 period. It competed directly against 4th gen systems such as the Master System and 7800 and actually outlasted every bit of its 4th gen competition, finally being discontinued in 1994.
The SG-1000, on the other hand, is not quite so clear cut. It came out in Japan on the same day, July 15, 1983, but was replaced on October 20, 1985 by the Sega Mark III, which is the Master System. The period it competed (1983-1985) is only the first two years of the Famicom's life and it was pretty well dead by the time the NES gained any ground on the US market. Is it 3rd gen, with the ColecoVision that it's based on, or 4th gen, with the Famicom it launched alongside? My personal opinion would be to put it with the 3rd generation. It had hardware that was similar to the ColecoVision and competed not only with the Famicom, but with the Epoch Cassette Visions, Bandai Vectrex, and Casio PV-1000. All systems that died out by the mid-1980s along with the SG-1000. I think a "proper" 4th gen system was still available new in 1987. 3rd gen only needs to make it to the video game crash. ColecoVision lasted 1982-1985. SG-1000 was 1983-1985. NES was 1983-1994 and far more powerful. The Master System, available 1985-1991, was Sega's true 4th gen system. It fought most of the battle against the NES. In fact in the USA it was Sega's initial offering against the NES.
The RDI Halcyon. This system was planned to launch in January 1985 in the USA (right in the ass crack of the video game crash). As for specs, I'm drawing a big fat WTF. It's not comparable to the NES, but it is comparable to some high-end ARCADE games from as early as 1982. It never competed against the NES - the system was DOA on arrival. In January 1985, the only video games available in the USA were used games (no doubt selling for pennies on the dollar) and a tiny trickle on Atari 2600 (a 2nd gen system!) and ColecoVision. Vectrex and Atari 5200 were canned in 1984. The video game market was 97% dead, so the Halcyon would have competed against... nothing. Still, I call it a 3rd gen system because it WAS based on laserdisc games that were in arcades at the time ColecoVision, 5200, and Vectrex were the hot new systems.
Dreamcast, on the other hand I still call 7th gen even though it competed mostly against 6th gen. 3 years passed between the last major 6th gen launch (N64) and only a year passed before the first 7th gen competitor came (PS2). But unlike the SG-1000, it was a major leap forward from the prior generation.