Random Enter The Gungeon Facts - Wasted Heart Pieces 

Oh so that's how that gumball machine-like heart dispenser appears!

Another tip I'd give is that, when playing as Bullet, and at the moment you're primarily using blasphemy, if you find a health pickup, BUT haven't found Bello (main shop) yet, leave it there. Incase unfortunate happens and you take damage, losing your ability to fire projectiles with Blasphemy, you can go back to that room and get the health, recovering the ability. Having to slog through multiple rooms to find the shop while being forced to get in close vicinity with enemies will most likely get you even more damaged, especially if you get a tough room. Ignore this in The Forge and Bullet Hell, since health stations will appear at the entrance once you save some health for later.

i always take the full heart piece by accident

Another fact that took me way too long to realize is that the heart dispenser shows how many hearts it has on the map in the bottom left. Very useful when trying to figure out how much health to sell to the vampire.

I had no clue that the rat won't steal full heart pieces

I always picked it up because I thought the rat would take it, however I never tested that theory so this is good to know now