Oh, man...I'm a study in what not to do...this is all my own mistakes.
1. Do not dose lanthanum chloride in the DT. Use the filter sock.
2. Do not add two denitrifying bacteria products at once, or dose over the recommended amount. If you do by accident, do an immediate partial water change and add an airstone or two until the water unclouds after the bloom the product(s) will create.
3. Do not add water without testing it first, especially if it's store-bought or if it's been sitting for a while.
4. Don't take a nap while making RO unless you have a good float switch.
5. Make sure you have a siphon break in place on your return line (I seriously flooded three rooms before I figured out why my sump was overflowing!!)
6. Do not put three electric blue damsels in a 29g tank.
7. Have a good lid on your QT, and in some cases even on your acclimation bucket!
8. Tuck some cash away for new RODI filters BEFORE you need them.
9. Don't leave someone inexperienced to watch your tank.
10. Don't start with anything less than a 29g tank. 40 is even better.
11. Don't touch a power strip you just got water in, or get too close with wet hands.
12. Don't leave a power strip where it can get water in it.
13. Use a GFCI, even if you have to have someone install one.
14. Try not to drop your light fixture in the tank...and if you do, pull the plug before attempting to fish it out.
15. Don't dump extra water from a fish bag over the kitchen sink when the fish is still in it.
16. Don't buy ANY animal without intensive research. And while you're at it, research the equipment too.
17. Always assume you are eventually going to upgrade. Size your equipment accordingly.
18. Don't buy dry goods from physical stores (unless there's a particular store you want to support.) Just about everything that's not live or frozen is cheaper on the 'net.
Amazon.com, Bulkreefsupply.com, and drsfostersmith.com are great places to look for steep discounts.
That's all I'm recalling right off. I'll come back if there's more.