Horizontal ridges on the nails of my thumbs. I was suspecting everything from vitamin deficiency to cancer, but it turned out to be self-inflicted. It was due to my tendency to tear the skin on my fingers when I'm anxious. Somehow it just never occurred to me, despite how my fingers looked.

Another, slightly awkward one is the so-called "after-dribble" many men deal with - you keep getting piss stains on your underwear after taking a leak no matter how hard you try to shake it dry. I dealt with it for years until someone online mentioned that pressing the area behind the scrotum forward helps "milk" it empty. That person has no idea how big an impact that single comment made on my life.

As for keeping houseplants, what I do differently now that keeps them not just alive but thriving is soaking them completely - basically letting the pot sit in water for 12 to 24 hours, then letting it dry out fully so the top soil is dry and the pot feels light before soaking it again. I have a plastic pot with drain holes inside a decorative clay pot without any. Whenever I water it, I take out a marble from the pot and leave it on the windowsill next to it as a reminder not to leave the plant sit in water for multiple days. This kills the plant.