Keep Circulating the Tapes: Despite being one of the most iconic arcade shmups, Raiden II has not had a rerelease since 1995 when it was included on The Raiden Project, which is long out of print. However, in 2014, after well over a decade of dealing with encryption issues, the game was finally brought to a working state in the MAME arcade emulator.
The Windows version of III was also this when it was released exclusively in Japan during 2006. However, the port got rereleased internationally through Steam service in 2014, EIGHT YEARS after its original release.
Sleeper Hit: As revealed in this interview, the original game was a low-expectation project only made because the developer's previous game flopped and a vertical shmup was all they could do with the alloted budget. The game initially sold poorly, but eventually ended up being very successful thanks to positive word-of-mouth.