Too much light - It is easy to roast plants living in terrariums. The glass can act like a magnifying glass and burn your plants. Temperatures can get really hot inside your jar and before you know it, your terrarium can get as hot as a sauna. Most plants just can't take this kind of heat, so it is best to keep terrariums out of direct sun.
Too little light - Most plants need some light to survive. While there are lots of great low-light plants, there is almost no such thing as a no-light plant. While you can use grow lights or fluorescent lights outfitted with one warm and one cool bulb, placement near a window also works well.
Too Close to Radiators - The heat of a radiator can kill your plants quickly. Think of how fast a car with the windows closed sitting in the sun can get hot. If you put your terrarium on or near a radiator that is what conditions will be like inside your jar and most plants won't like that.
Letting plants get scraggly - Keep an eye on your terrarium plants and when they get leggy, prune them back. To keep plants small you can also root prune them.
Leaving dying plants - If a plant is looking bad in your terrarium, get rid of it fast or remove dead or dying leaves. If the plant is diseased, take it out immediately as it can infect other plants. If a plant is just plain ugly, it can ruin the look of your whole terrarium so take that out too. Simply dig the plant out with a small shovel, terrarium tool or long spoon, being careful not to disturb the roots of other plants (as much as you can). Replace the plant with one of a similar size and light requirement, making sure to surround the roots with soil, leaving no air pockets.
Dirty glass - Every once in awhile you will want to clean the glass of your terrarium both inside and out. I use a damp piece of newsprint or a lint-free cloth. Do not use any harsh cleaning products on the inside of the terrarium because it may present a danger to your plants.
Over watering - It is easy to over water terrariums. One way to prevent it is to use a spray bottle instead of pouring water. It is easier to lightly water if you spray. If you do over water, try to absorb any extra with paper towel. Leave the top off your terrarium until it has dried out.
Over fertilizing - Most terrariums do not need any fertilizer at all. Because you want to keep your plants small, you shouldn't feed them, which will cause new growth and the plants will quickly outgrow their confined space.
Choosing the wrong plants - While it is possible to grow almost anything in a terrarium, it is important to choose plants that will thrive in the type of terrarium you are creating. If you are making a closed terrarium, choose plants that like to be moist. Also make sure to choose plants for the amount of light they will be exposed to. Low or medium light plants generally work best, but make sure if you get a medium light plant, you are actually giving it that light level.
Growing succulents in closed terrariums - Succulents generally thrive high light and low moisture environments. If you put them in a closed terrarium, it is usually far too humid for most to thrive. You can solve this dilemma by simply creating a glass dish garden without a top. Note that even a large jar will be too humid - you want air to be able to circulate around your succulents.