After the beginning area of the game, the player can obtain a tiny music box from a child. Playing the tiny music box during the Father Gascoigne boss fight will cause him to stagger. Playing the music box near him when he's summoned will make him chuckle.
Contributed by DrakeVagabond
It is heavily implied that the pebbles are petrified eyeballs.
The most obvious indicator is that the pebbles are spherical and appear to have a hole the shape of an iris. It is also worth mentioning that all crows in the game drop pebbles, and that crows are known for eating eyeballs.
Contributed by DrakeVagabond
In the Hunter's Dream, there are books in the workshop titled "How to Pick Up Fair Maidens".
Contributed by DrakeVagabond
Before the boss fight with Spoiler:Gehrman, it is possible to force PvP in the Hunters Dream.
Contributed by DrakeVagabond
After reaching 40+ Insight, the music in the hunter's dream will change.
Contributed by DrakeVagabond
The boss Martyr Logarius uses a sword reminiscent of one of the variations of the sword Soulbrandt, an iconic blade in Demon's Souls.
Contributed by the_op_rainbow
A rare item called the Ring of Betrothal can be obtained inside of the chalice dungeons. Having the ring in the inventory unlocks the option to "Propose Marriage" when talking to Annalise, Queen of the Vilebloods. However, Annalise will reject the offer.
Contributed by G-Haven
As the player progresses though the game, the number of messengers in the bird bath at the Hunters Dream will increase and they will be holding items associated with the badges the player has acquired.

Additionally, the number of messengers around the lanterns will change based on PVP. If the player plays offline, only their four messengers will appear.
Contributed by DrakeVagabond
The Doll in the Hunter's Dream will respond to different emotes, yielding different reactions for certain emotes.
Contributed by DrakeVagabond
In the Chalice Dungeon, there is a suit of armor lying on the wall in one boss arenas. This armor is the iconic Fluted Armor from Demon's Souls, positioned in the same fashion as on the box art.
Contributed by DrakeVagabond
During the game's public alpha test, a player managed to gain access to a restricted area and was killed in an unfinished boss encounter with Father Gascoigne. When dying, the boss muters the words "Umbasa", a common word used by NPCs in Demon's Souls, Bloodborne's spiritual predecessor. This may suggest that Bloodborne is set in the same universe as the Demon's Souls series or simply a reference to the series by the developers. This line of dialogue was removed from the final release.
Contributed by MIXER475
If the player leaves the game running idle for 12 hours or more, the game's difficulty will drop. This is caused by a glitch in the game's code that causes bosses to only load a limited number of their attacks as a side effect of a memory leak.
Contributed by DrakeVagabond
Screenshots of the game were leaked before the official reveal, under the title of Project Beast. This was believed to be connected to Demon's Souls. However, director Hidetaka Miyazaki has since stated that this was false, due to Sony wanting a new IP exclusively for the PlayStation 4.

Miyazaki also stated that he had always wanted to create a game set in the Victorian era, but felt that the video game consoles at the time were not powerful enough for what he envisioned. According to producer Masaaki Yamagiwa, the game was locked to 30 FPS due to the team believing it to be the best for action games. Miyazaki later stated that the decision was made because of technical limitations.
Contributed by DrakeVagabond