I'm of course referring to Wan Shi Tong's library which was destroyed by Avatar Aang following he and his party's incursion into the library.
And then there's also the after effect of Wan Shi Tong barring entry of any human scholars into the ruins of his library after Avatar Korra's opening of the Spirit Portals. Which can also be attributed to Avatar Aang as a reactive effect by Wan Shi Tong.
You can compare it to Alexandria's library. Didn't exactly set back advancements however it is quite sad and at times frustrating having so many ancient pieces of work gone and destroyed. Luckily for the avatar world, theirs merely moved to the spirit world which now can be accessed by any human as well as the avatar (well as long as they are willing to enter the spirit world and find it). For the case of Alexandria however, it is completely inaccessible...