Most people have a fascination with the unknown and the taboo, and in 2015, a 4chan user posted about a mysterious OVA in which nine girls, one of which, was named Saki, were trapped in a bathroom with no exit. Over time, they come to realize that the only way out is through suicide. Gruesome deaths, screams, and traumatizing scenery accompany the 30-minute video until it suddenly stops.
Other users began adding to it and claiming that they watched the video themselves. Some recalled the girls talking about their lives before their deaths, others said that they fought first. Three years after the fact, another user on Twitter described the OVA, but with it having three girls instead. Regardless, all agreed that there was a scene in which one girl helps the other drown herself, and the name of it was somewhere along the lines of, Saki Sanobashi.
Theories
Some theorize that the work could be reminiscent of Shoujo Tsubaki by Suehiro Maruo, an eroguro manga from the 1980s that would, later on, be adapted into an OVA series. Additionally, it's said that most of the footage from the OVA is either banned or destroyed due to its disturbing nature. If that's the case, then it's not too far off to say Saki Sanobashi might have a similar predicament.
There were also comparisons to Dragon Head, another manga from the 1990s with similar existentialist and gorey themes - however, there was no distinct scene with nine different girls in a bathroom. There's also the theory that the OVA might have been loosely based on the manga, instead, though Dragon Head was released after Saki Sanobashi.
But it wasn't just the English-speaking side of the internet that knew of this legend. Online users from Spanish, Portuguese, and even Greek-speaking countries claimed to have seen Saki Sanobashi. One user from Macedonia says the name they knew it as was in Japanese, ナイン角度 meaning, "Nine Angles". Though this could also be read as, "Nine Degrees" or "Nine Corners". This may have been a fan-name, as another theorized that the actual name may be related to The Lady of the Sea of Blood, called Blood Punch of the Lady.
The biggest clue, however, lies in the title itself. In Japanese, there are three different alphabets and the third one is infamous for having multiple characters that are read the same way, but have different meanings. Given that Saki Sanobashi was clearly aimed towards adults, the third alphabet, being kanji was most likely used in the title.
This is further backed up by other users across the net claiming that the title of the anime they saw was indeed in kanji. An alternative name that users theorized is that it's actually called 先延のばし, which is read as, "Saki-san no bashi", meaning, "Saki's torture room". The word, "Sanobashi" comes from a bridge located in Gunma, so chances are, OP just misremembered, "Saki-san no bashi". If this is made up, the original poster must be familiar with Japanese enough to "misremember" such a unique title.
Does It Really Exist?
Speculation surrounding the OVA brought into question if it actually exists. In 2019, there was a Reddit user who claimed to be the original poster of the Saki Sanobashi thread. According to them, it was a joke that they made up for shock value. However, there still remains speculation. Because all 4chan users are anonymous, there's no proof that the redditor was actually telling the truth. Additionally, multiple people had claimed to see the OVA as well - though there remains a possibility it was the same person adding onto it, or they were just playing along.
After some digging from another person, they found a website with a deleted video and a description matching that of Saki Sanobashi. Sure enough, it had a production date, 1986 - which adds up with OP's claim that the OVA was clearly animated before the 2000s. However, the supposed site that it was available on wasn't created until late 2014. But that doesn't mean it didn't exist beforehand. While it was owned by another group, there still were streamable videos - though Saki Sanobashi was far from the type to appear on the site. But those on the internet are known to sneak content where they don't belong.
The early 2000s had a lot of obscure taboo anime, even manga scans were shared around on online forums. Since this was the early age of the internet, having backups were pretty rare. Additionally, because this was such a popular fad, there's no doubt it garnered enough attention for the authorities to take it down. Even in Japan, infamous guro footage was trashed and banned by the government, itself. These were so rare, no one ever knew the names of them.
Though it is possible that Saki Sanobashi has the same dilemma as The Clock Man, which was a scene from a broader work that people swore up and down they saw, but could never find until recently. There was another disturbing video, previously mentioned, Lady in the Sea of Blood, that was rumored to exist but was ultimately resolved to be fake - until the video surfaced in 2019.
The concept is vague enough to be a legitimate horror anime, but specific enough to last an impact on the viewer. As mentioned previously, some YouTubers theorize that it's actually a scene from a bloody horror anime, and they just misremembered it. This could be the most likely scenario, as a "bloody bathroom scene between school girls" isn't a unique concept. 2018's Magical Girl Site had a scene where the main character was being cornered by her bully, who attempted to slice open the inside of her mouth.
Thanks to her friend's intervention, the bully ended up slashing her own throat instead. A rather disturbing scene for an anime with cute wide-eyed girls in school uniforms, needless to say. Gakkou no Yuurei, another OVA has an infamous bathroom scene where the school janitor is hunted down and killed by the ghost of a schoolgirl.
As of 2022, YouTubers and theorists alike are still bumping their heads together on if this actually exists. Perhaps we'll never know.