I haven't made horrible investments equipment wise due to 2 awesome LFS I shop at. However, livestock wise I have. I would usually buy my livestock from Big Al's and not those two other stores before due to the fish being cheaper. What I didn't know was that these stores usually got the more sub par fish/sick fish and so the only reason they survived in their tanks was cause they would heavily medicate their systems with copper etc. These fish carried Marine Ich, Marine velvet, Brooklynella. Which means my tank with none of the medications equals dead fish in a month or less. I couldn't understand why I was losing fish so fast. I even started to quarantine fish but they would die from bacterial/fungal diseases like popeye.
I left my tank fish less for 8 weeks and after started to buy fish from the other two stores and fellow reefers. Haven't lost any fish or livestock of inverts after that. One of the store owners of the 2 awesome LFS told me he purchases the most healthy and lively fish. He also doesn't medicate tanks and believes that stress free environments are best for fish and key to fighting diseases.
Moral of the story, don't buy cheap, get the vary best in this hobby.