1. You buy a dress right off the floor even though it doesn't fit you great just to save money. You think, I'll get it altered and it will be just fine. This is not the case. Alterations can actually cost more than your dress - I promise. So unless you know someone who can alter it for you (altering a bridal gown is not like altering a pair of pants), then stick to finding a dress that fits you or you can order in your size. This also goes for picking a dress off the floor that you like, but don't want that there, or want the train cut off, or want something added. This all costs money - and usually at a higher cost than you think. You are better off finding a dress you like as is.
2. You buy a dress because it's a good deal. You go into a bridal store and find a dress that is a good price... you order it in and when it comes in you try it on and break out into tears. You know it's the not your dream dress and immediately panic knowing you will not be happy with this dress. The problem is you can't return bridal dresses. Once you buy it or order it in, it's yours. So now you struggle and probably end up buying another dress because you just aren't happy with your "good deal".
The best thing you can do to find your dream wedding dress is figure out what your vision is. Some brides know they want to feel like a princess in a full ball gown, and others know they want to show off their curves in a mermaid dress. If you aren't sure - flip through some magazines - get an idea on what your eyes gravitate to. Go try dresses and find out what styles fit your body type best. Once you know what you want - don't stray from that. The best way to save money is to know what you want and find it.