Have you told a story which required additions to its core for the purpose of believability? I'm not speaking of a lie, but for the purposes of this description, we shall go with that.
Say your supervisor wishes to add after hours work into your schedule. You've taken on the role of two additional coworkers per company-wide layoffs and a temporary pay reduction. You are "a team player" but more is asked. Turning down the additional work puts you in line for termination. Accepting isn't an option. If you have an excuse, you may simply swerve for a day. Make the excuse good enough and you bypass not just this request but future requests on your time.
ie/ So sorry boss, mum is in hospital. I must go for a visit immediately after hours. You understand as your wife has been ill.
Problem. You now have sick mum. What to do when holes are found? "Mia, you went to the bar with L. I saw you. Thought you had to go to hospital after work. Is Mum better?" You grab verbal cement to seal hole. "They put a tube down her throat and she's very sore. I promised to go Saturday instead." Then boss wants to know what the tube is and why (note: tears and loud sobbing only work for a short while). You make up a disease and suddenly you have logic leaps to rectify.
Similarly, writing can create these monstrosities of logic. Claimed by Darkness was one such. One hero, a vampire, falls for another.
What keeps them apart?
Hm. Ah-ha! Other hero is godlike to the vampires.
Um. Because he's the answer to prophecy and becomes a werewolf.
But if he's a werewolf, won't that create mating issues and aren't werewolves and vampires generally unfavorable toward one another?
Yes, I see what you mean. Right, werewolf "god" is also the answer to werewolf prophecy and prophecy dictates he unite both races!
Bugger. "God" is a scientist and-um-and he creates a hybrid of the two races.
You said four chapters past that vampire blood is toxic and werewolf blood from a full wolf can sicken a vampire. How will you combine them if both bloods are predatory and toxic?
You see the problem? We play cat and mouse with our fictional realities, do we not? The "what if" questions that fill out the story are the same that will destroy it if the author is unable to find the appropriate fictional cement. It distills, hopefully, into a story wherein the reader can suspend her disbelief:
"Mum is in hospital with tube down her throat. She is very sore because a vampire mosquito bit her uvula. Without locating the exact miscreant mosquito, she will labor indefinitely on the shadow edge of death. Therefore, sir, I cannot take on any work which would delay my time. The creature was last seen at the bar on Hennepin Ave where I will look for it every Friday night until exhausted. Selflessly, I shall care for my ailing mother in hospital all other days after I have devoted my day to you and this esteemed institution. I'm sure you understand as your wife has also been ill."