BIFIDOBACTERIUM...
So did our favorite bounty hunters parish from the poison in Session #11? What was that... thing?!
Well, when Spike was trying to identify it he cam upon a close match, it is just that he got the mirror image when trying to make the match. He identifies it as bifidobacterium, which is actually healthy for humans! Check it out here.

However...

Alicia, a vigilant fan, points out that most organic substances have an asymmetrical arrangement. Since the the digestion of food involves complicated chemical reactions between asymmetric food and asymmetric substances in the body, there are notable differences in taste, smell and digestibility of mirrored substances most of the time. The Bebop may have encountered the opposite of bifidobacterium, which could very well be a hazardous substance. Since bifidobacterium controls intestinal pH levels, the opposite of bifidobacterium would probably cause more acid in the body.

THE BETA TAPE
Who sent Faye the beta tape and deck in Session #18? 
It could be anyone's guess. A theory has been circulating that Faye sent the tape to herself and that it had been circulating the Solar System falling into the possession of women with similar names. Another theory suggests that the tape, a time capsule for Faye, has become a time capsule for all of humanity in space, just randomly sent to various people in the Solar System. Yet another theory is that an ex-boyfriend sent it to her to repent for past misdeeds. I personally believe that it was sent from an old classmate.

THE LOVE TRIANGLE
Why did Vicious not kill Spike in Session #5 or Session #12? 
My take is that both Spike and Vicious were waiting for the right moment to end each others' lives. They were both waiting till they both had nothing to live for. Vicious had already lost his reason to live when he gained control of the Red Dragon syndicate.

ENLARGE

RED EYE ON THE TABLE
Flashback sequences in Session #5 and Session #13 show one shot briefly on a Red Eye vile and device on Julia's table in her apartment. Who was using it? The popular vote goes to Vicious. Thanks to Michael Wayne for pointing this elusive one out.

IS SPIKE REALLY DEAD?
Even though he managed to survive terrible gunshot wounds and being thrown out a cathedral window, an efficient experimental killing machine and a deadly bio-terrorist  among other things, Spike was "already dead" since Julia disappeared three years ago. Julia became his life, he had "died" when she left. The days on the Bebop are his purgatory, his "belly of the whale" where he aimlessly lives as if in a dream. When he loses Julia indefinitely in Session #26, he decides to end his feud with Vicious. Faye asks him why he chooses to meet death. He responds saying that he intends to find out if he is alive, not to die. In the end, he finds out that he was "dead" all along.

BULL & THE GREAT SPIRIT
Did Bull know from the first session that Spike was going to die?
"You will meet a woman... the woman will hunt you... and then death." This could be referring to Spike's final encounter with Julia.

Did Spike join the "Great Spirit?"
As for the "Great Spirit," Spike must have joined it. Gren had failed because he feared death, not able to let go of his ties Vicious and Titan. Spike severed all ties to his mortal life and did not fear death when it approached. Gren's star fell, ending prematurely. Spike's star faded away as if it burned to its fullest extent.