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.
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.Pay attention to that last one ya'll: yes those succulents look cute in an enclosed terrarium with your moss, and no they will not last. Why make a desert themed open-top dish garden with sand for your cacti?
Common Terrarium Mistakes by Kerry Michaels, Container Gardener Guide, about.com