First up, absolutely no shade to anyone for doing these things because they are EASY mistakes to make, especially if they are new to the houseplant world -- and you can let me know if you've ever made these mistakes! (Spoiler: I have!)
1️⃣ Overwatering 💦
Honestly, this is almost everyone and the name is totally misleading. Overwatering is NOT giving your plant too much water -- it's watering your plant too often. It's not about quantity, it's all about frequency!
Most houseplants need to dry between watering sessions but some do prefer moist soil so it's still always best to a) research what that plant wants and b) check the soil before watering, never water before touching that dirt!
2️⃣ Underwatering 🌿
The next biggest mistake is not watering THOROUGHLY enough. Some folks try to avoid overwatering (mistaking the meaning) by underwatering ie. giving their plants just a little water at a time, instead of a full soak when they need it.
For plants that want to dry out, you need to fully water them when they are dry and thirsty. I'm talking a serious soaking, watering pouring out the drainage holes, big ol' drank.
3️⃣ No Drainage 🪴
You will have a much easier time if you let your plants have drainage. Either leave it inside the nursery pot and place that inside a decorate potcover or use a decorate pot that already has a drainage hole.
Rocks in the bottom of a pot don't offer the same drainage your plants need. You can end up with root rot quickly if water is constantly sitting at the bottom of your pots.
Growing houseplants and building a collection of thriving greenery is a learning experience just like any other skill. I don't think there's anything as a green thumb -- anyone can learn to care for plants when you have the right ones for your space and your lifestyle!
Everyone's going to make mistakes so don't worry about it at all! Start small, grow your plants and your skills at the same time, and know that anyone you see with an amazing plant collectoin has definitely had some duds in their day!