This happens; a user defined-field will someday get "screwed up". And it will be an important one. Like one day, Customer Type will magically switch from "Customer" to "Vendor". Or a complete street address will go missing. It is inevitable, certain as the fact that no one will fess up to it.
If this happens a lot, it might be worth enabling Logging Changes to History. This is done on a per-field basis and is done in Field Properties;
1. Right-click the field in question, select "Properties".
2. Click on the "Security" tab.
3. Check the box labeled "Log Changes in History".
4. OK your way back.
Every time this fields' value changes, a History item will be written against the Contact Record. It actually tracks a lot of detail about the previous values, etc. You may find that it "Junks Up" the History tab a bit, and may only wish to keep this enabled until you figure out what's going on with your field updates.