There are many ways  to build a solution in FileMaker, especially moving forward into FileMaker 7, 8, 8.5 and 9.0. Over the past few months we have been seeing more and more requests for repairs on other developers work, or to add on to an existing solution. FileMaker is such a powerful tool that there is not necessarily always one way to build a database, that being said there are many ways not to do things. 

I think the most common mistake of the general FileMaker Developer community is the coding into a corner scenario, I have been there myself and it is a real pain to fix sometimes. It is always best to really plan things out and have a good design that is forward thinking; ideally a development template to be the foundation. If the developer fails to plan things out you will often find yourself having to rebuild or replace entire systems. Now at some point in time most database solutions do require significant rewrite or complete rebuilds. Without a good development plan or blue print to reference the rewrite or rebuild can be quite time consuming and expensive. So my point is plan, plan, plan.. the extra time it takes to do so is worth its weight in any commodity.