Ketsui is a vertical arcade shoot em' up developed by Cave with an interesting story behind it. Released in early 2003, it's one of three games (three and a half if you hate whole numbers) released in 2002 and 2003 for Cave's custom IGS PGM hardware. All three games, Dodonpachi: Dai Ou Jou, Espgaluda, and Ketsui, are considered to be of excellent quality and are extremely popular even today. A japanese developer named Arika made plans to port all three games to the Playstation 2. Dodonpachi:DOJ and Espgaluda were released and praised by the community for being extremely accurate and faithful ports for their time, but the Ketsui port was scrapped. According to Arika, there were problems emulating a certain part of the game. A section in Stage 5 involves flying down a mine shaft. The illusion of the player descending was achieved by swapping the image in the background plane and scrolling the new image in reverse, and then swapping to yet another background plane at the bottom of the shaft and reversing the scroll direction a second time.
The aforementioned problematic segment is at 3:20
Apparently the PS2's hardware was incapable of managing this at an arcade perfect framerate and the section suffered from terrible slowdown. Unsatisfied with the port's single imperfection, Arika opted to cancel the title entirely rather than release what they viewed as a flawed product. Apparently the port was otherwise perfect, and Cave gave them the go-ahead to ship the title with the slowdown intact, but Arika refused, and so Ketsui went without a port, and remained out of the reach of western fans unwilling or able to spend thousands purchasing the game's arcade PCB for the better part of a decade.
In 2008 japanese developer 5pb announced plans to port Ketsui to Xbox Live Arcade, but Microsoft rejected the title, claiming to want to cut down on the number of arcade ports on XBLA. Ketsui was finally announced to have a full on-disc release in 2009 but was pushed back another year, being released in 2010 instead.
After seven years of elusive existence, the game was finally available to anyone with a 360 capable of playing NTSC-J games. During those seven years, Ketsui garnered status as one of the best games of it's type, strongly praised by the few who had played it, and remaining one of Cave's most highly rated titles after it's console release. It's also properly emulated in MAME now, further increasing it's availability to western audiences.
There are a few lessons to be learned from this story. One is that Microsoft sucks because they hate arcade games despite having a service called "Xbox Live Arcade". Another is that apparently the PS2 is kind of a shitty console, because it was incapable of emulating a game that was developed for and ran perfectly well on a printed circuit board with only 20Mhz of processing power. The third and final lesson is that they don't make game developers the way they used to. It's hard to imagine any studio these days working as hard as Arika did back then to perfectly emulate Ketsui on new hardware, and even harder to imagine that studio having the balls to dump the entire project because they were unable to accurately recreate a single tiny section of the game. That is a testament to perfection, my friends, and one that you are unlikely to see in an age with humongous zero-day patches and Silent Hill ports without any fog.