One of the first acronyms to come from the computing world was GIGO: Garbage In, Garbage Out.
Agnes Riley has a very helpful post on the subject of clean data, and some tips for developers on how to keep the problem under control:
This is something I’ve bee dealing with for years. It hit me like a large hammer in the chest when one of my users replaced the contents of one field in about 5000 records with HTML she copied from the web. This was done accidentally, and clearly she felt so bad that she didn’t even tell me. This happened about 9 years ago. I realized I had to put in some measures so this won’t happen EVER again. Now, you can think of every possible measure you can take and users can still surprise you. It’s similar to spam online. We keep getting smarter at how to deflect spam but spammers are always a step ahead. Below are some things you can put in place to make sure your (their) data is as clean as it possibly can be
Study the data entry process when designing: If copy and paste is part of it, be sure to clean the data after it is entered. Your clients will appreciate it come report time.
More...Clean Data in FileMaker