In the demo, it was possible to have Reimu B's orbs linger by bombing just before the stage ends, on the boss's last spellcard. Also, it was possible for Sanae B to inflict bomb damage on a spellcard by bombing on the non-SC before it. Both have been fixed in the final.
There was a glitch that caused the game to crash when the score counter reached one billion points, be it while playing the game or while watching a replay. This has been fixed as of the v1.00b patch.
By skipping through the dialogue at the end of a boss fight it is possible to end the stage before the items and star piece are collected by the player.
If a player's score exceeds 2147483648 (231) points, an "x" is placed beside the Score and High Score text. Any more points earned will be subtracted from the score shown by the score counter. The player's actual score is unaffected.
Difficulty Level Names
Like most of the other Windows games, each of the difficulty levels has a special name and a short summary.
- Easy - For Sunday Shooters - For humans with busy lives.
- Normal - For Active Gamers - For people who play this sort of game because they like it.
- Hard - For Residents of Gensokyo - For people who don't fit in well with the real world.
- Lunatic - For the Lost Race - For people who insist they are chosen ones.
- Extra - For Youkai Extermination Specialists - For people who would never be afraid if they met a youkai.
- Some fans speculate that TH12 focuses on UFOs because the year before its release was the 30th anniversary of Space Invaders. The fairies on stage 6 move like the invaders from that game, and the enemy UFOs on Extra show hit scores like in Space Invaders when they explode.
- Some fans also believe ZUN's character art looks far better in this game than in any of its previous appearances, marking it as an improvement in ZUN's art style.
- There are numerous references (albeit no direct connections) to the PC-98 games, possibly more than any other Windows title.
- UFO is the first Windows game (excluding the spinoffs) in the series to have no midboss only characters.
- It is also the first Windows game to start off an Extra Mode stage where enemy shots are not directly aimed at the player.
- This game would be the second Windows game ever to still have red powerups, even if the player is at full-power. The first being Embodiment of Scarlet Devil.
- This is the only game where every character art is holding (or has) an inanimate object.
- In the original version of Undefined Fantastic Object, the UFOs were colored red, green, and blue. However, due to possible difficulties colorblind players could have differentiating between them, the 1.00B patch slightly adjusts the color of the red UFOs. It also solved a bug in which the player could not exceed one billion points.
- ReimuB's bomb, Fantasy Seal, can cause a very slight amount of damage to Nue during a spellcard at the end of her invincibility frames. This is most likely due to the splash damage of Fantasy Seal extending beyond those frames.
- Like Embodiment of Scarlet Devil, all of the bosses and mid-bosses debut in their game.
- Return to Undefined Fantastic Object