These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Spud and the hilariously disgusting bedsheets incident, which results in him accidentally showering his girlfriend's family with his own piss and shit.
Renton shoots a sleeping dog in the nuts, with an air-rifle. This causes the dog to wake up in a rather bad mood to say the least.
Renton and Spud casually walking into a nursing home amongst the old people and simply carrying away the TV. Irvine Welsh spent time with junkies before writing Trainspotting. That was a true story.
The job interviews, when Renton and Spud try to become "normal" hard-working people. Renton is hilariously clueless in his, while Spud (who overcomes his shyness by taking speed) blathers fast and incoherently about completely random subjects.
Arguably any of the main four (including Begbie) among each other.
The Woobie: Poor Spud, other than his self-destructive drug habits he doesn't do anybody any harm, yet he gets sent down while the considerably more deserving Renton escapes prison by enrolling in rehab in a quite cynical attempt to make the judge think he's attempting to reform of his own volition, and then he gets his palm sliced open by Begbie and nearly bleeds out, while his so called mates refuse to call an ambulance for him
Jerkass Woobie: Tommy, who ends up getting AIDS from using heroin after his girlfriend dumped him (though notably he uses it far less than some of the other addicts who are still in fairly good health). Not only is he dying, but he ends up with hate messages scrawled all over his walls too. His acceptance of his fate and lack of hatred for the guy who gave him that first shot ( Renton) makes it in some ways a Crowning Moment of Heartwarming, but all the more tragic that such a nice guy should end up this way.
Memetic Mutation: The "Choose life" opening spiel was much imitated in Britain for a while after the film came out, but not as much as the distinctive poster.
Misaimed Fandom: For a movie with such unsympathetic portrayals of addiction, it has an awful lot of fans in the drug community.
Plenty of real-life opiate addicts are big fans of this movie for several reasons: one, it's about as accurate as movies get in its portrayal of the heroin experience, ups and downs both, and two, it's not overly anvilicious about it in the way that, say, Requiem for a Dream is; it takes more of a neutral, observational stance, rather than outright saying "drugs are bad and everyone who does them will inevitably die horribly" or "hey, drugs are great and nothing bad ever happens to people who do drugs!" It's simply an objective, realistic look at addiction that doesn't try to patronize or hammer a moral into your head.
Begbie has one in some quarters. You're supposed to see him as a violent, empty sociopath. Others see him as Crazy Awesome, especially immature readers/viewers. Welsh himself actually finds him scary to write.
Moral Event Horizon: Begbie's treatment of his girlfriend June. Who is also the mother of his unborn child.
Nightmare Fuel: Renton's dream sequence when his parents lock him in his room to get him off heroin. The baby in particular.
It's quite a lot worse in the book. Welsh's writing style is extremely visual and truly horrifying in its description of the hallucinations.
Narm: In the movie, baby sequence may have been a bit too exaggerated to be actually scary. YMMV, naturally.
Older Than They Think: The concept of waiters (or waitresses) tainting the food of customers they dislike is probably most closely associated with Fight Club, but this did it first.
Tear Jerker: Tommy's decline into heroin addiction and eventual death. In the book, we are never shown his death scene, but Renton visits him for the last time and finds out he has AIDS. Renton knows that Tommy won't survive the winter.
Baby Dawn's death. Sick Boy breaking down in tears also counts.
Sick Boy: Say something Mark...FUCKING SAY SOMETHING!