What are the reasons for data loss in Salesforce?

Data loss may cause due to following reasons:

• Changing data and date-time
• Altering to percent,number and currency from other data types
• Changing from multi-select picklist, checkbox, auto number to other types
• Altering to multi-select picklist from any type except picklist
• Changing to auto-number except from text
• Changing from text-area to e-mail, phone, URL and text

Changing field type could cause the data loss. So we need to take backup of data first before doing so.

Adding some points:

There are 5 main causes of data loss in Salesforce:

1. Accidental or malicious deletion can occur when a user inadvertently or intentionally deletes one or more records. An individual record can easily be deleted by clicking on the delete button located on an object page. Multiple records can be deleted at one time with the mass delete utility
2. Bad code can make incorrect changes to many records. Developers plan and test their code to avoid mistakes but program errors still happen. Not every actual scenario is always covered so code can sometimes have unexpected results. These changes can be very difficult to identify and even harder to fix.
3. Data import gone wrong will have a far-reaching impact. Tools like the data import wizard and data loader allow users to update records in a single operation. Unfortunately an incorrect mapping can yield bad values in a large number of records.
4. SOQL mistake can happen in an instant. The developer console query editor can be used to execute queries and the wrong syntax can create a serious problem. An update query is designed to potentially impact large number of records so a backup should always be run prior to any mass update.
5. Problems with an integration could inadvertently update the wrong records. There are some very powerful integration tools for Salesforce that can update hundreds, thousands or even millions of records through automated processing. Without a current backup, an integration error of this magnitude could be impossible to recover from.

Thanks