There's a ton of interesting tidbits of information floating around about the game, always being shared in hushed tones in secluded corners of the fandom. Here's a growing collection of information I've found that the average fan might not know about!

Note: Sources for all these claims will be coming in the future, I just don't have all my links organized at the moment... For now:

• There was going to be a spiritual séance feature involving Grace in Pathologic 2, but it was scrapped due to it requiring too many additional resources to implement. [x]
• There is a "secret" alternate ending cutscene on Day 1 of Pathologic 1 if you fail the day's main quest. (This may not be entirely unknown, but I didn't discover it until I went for all achievements).
• There are different editions of the tabletop game with alternate art for some of the cards. The portraits for the three healers in particular are different.
• The "portrait quotes" available on the Wiki are actually in the game itself. You can read them while in dialogue with a character and clicking on their portrait. (I didn't know this, so I'm including it.)
• Pathologic (2005) was released for free as a CD included with the April 2007 issue of German magazine Computer Bild Spiele. [x]
• The item description for Shoes in The Marble Nest reveals that Daniil had worn down three pairs of shoes getting to the Town, implying he made the entire journey on foot.
• In Pathologic 1, Artemy can hear voices near specific locations in the Steppe and behind the Warehouses. Apparently, this is an easter egg of Nikolay Dybowski reading the poem "The Ratcatcher".
• According to an interview with Andriesh Gandrabur, he and Dybowski disagreed on the style of music that should be in the game. Dybowski preferred an atmospheric sound, comparable to the Pathologic 2 soundtrack, whereas Gandrabur preferred dark electronic music. In the end, he sent the music directly to the technical director, bypassing Dybowski, so that his own vision of the soundtrack would be put into the game. (Apparently, Dybowski didn't even like the main menu theme.) [x]*
• Despite what people think, there were two different main artists who worked on Pathologic: Irina Kapitonova and Meethos. You can tell their artstyles apart by Kapitonova's more grounded, down-to-earth style and Meetho's more stylized and avant-garde outfit designs - not to mention their signatures.
• The Town that the game takes place in is never explicitly named in the source material. It is colloquially known as "Town-on-Gorkhon" by fans, but is not confirmed as an official name.
• The Steppe glyphs/Longmark symbols that show up in the corners of the screen in Pathologic 1 actually indicate when you have received a letter or when an update has been made to the diary/quest menu. It's unclear what the sigils are called or what they mean in the Steppe language itself.
• Certain aspects of the 2005 English release were censored to avoid an age rating increase. Lika was replaced with a small man named "Some git", and a few lines of dialogue with Notkin are changed to reflect this. In addition, the player is unable to kill Boy NPCs.

Spoiler-y stuff:

• Dybowski confirmed that the Teensy and Tot sitting with Notkin in the Haruspex's ending are The Powers That Be.
• In a dialogue between the Inquisitor and Daniil, it is revealed that Daniil had successfully reanimated a corpse during a lecture, with the Inquisitor being one of the witnesses.
• Artemy had an older brother named Ersher, but he died young. The circumstances of Ersher's and his mother's death are not explored in the story.