In Episode 1 (and Tape 1, Side A) Hannah says that there are 13 sides to every story, and that this is the reason for making 13 tapes. However, at the time she recorded the first tape, she had only planned on making 12, which she reveals in Episode 12. She didn't intend to record a conversation with Mr. Porter until days later.
Jeff's accident is ruled to be the result of drunk driving because of the presence of alcohol in his car. However, in the case of all traffic accidents resulting in death, thorough investigations are always conducted, including toxicology reports. Jeff would undergo a mandatory autopsy to determine the exact cause of death and therefore it would be known that he was not intoxicated at the time of the accident.
When Justin smokes heroin he mostly smokes it by straight inhaling the smoke coming off of the foil by putting his mouth/face to/into the smoke. But in reality to smoke heroin you have to use a tooter. Which is usually a hollowed out plastic pen tube. There is one part where he has/ uses a tooter to smoke it. But in the rest of the show he smokes it incorrectly without any tooter to actually catch the smoke. With most of the smoke just going up into the air. In reality if you tried to smoke heroin like he does without a tooter of some kind you would miss all the smoke, wasting it and not really getting high from it.