1. "Microbreaks" from work - grabbing a snack, browsing random things, checking the news/stocks/how far below $4000 BTC fell, etc. This is mainly harmful because it keeps you from doing work fast. The, arguably, worse part of it is that you get nothing done in a long time and then feel like you "worked all day" for nothing. So you waste time doing something that isn't pleasurable, but you get far reduced benefit, making it seems like the productive task wasn't worthwhile at all. The excuse is that it breaks you out of being stuck in a rut - a lot of the time while coding, I will stare at some problem endlessly, when I should have stepped back and changed my approach. Herein lies the conflict between "deep work" and "stepping away to get perspective" for me. It seems both have merit, and it takes experience to know when to do which. With that said, switching context constantly does NOT give your mind that break it needs, because the mind becomes engaged with something else, rather than continuing to work on the problem.
2. Sleep 7+ (preferrably 8) hours. At this point it's well known that sleep is essential to memorization, concentration, testosterone production, and athletic recovery. "Working hard" by avoiding sleep undercuts your efficiency and quickly starts to have negative effects.
3. Eating sugar. Rots teeth, makes you fat, spikes blood pressure and increases the likelihood of heart disease (https://www.health.harvard.edu/blog/eating-too-much-added-su...). All for no nutritional value. The twist here is that "it makes you feel good" and "gives you energy". The "makes you feel good" part is definitely a habit that can be broken. The "gives you energy" part. Well, that's something that I actually think happens. If I drink a KickStart, I can stay awake through sprint planning, which is quite a feat. I haven't found the blood sugar spike & drop to be true in my experience, which makes this a difficult habit to fully break for me.