What can cause data loss in Salesforce?

Hi
Data loss may cause due to following reasons:
1:-Changing data and date-time.2:- Altering to percent , number and currency from other data types.
3:- Changing from multi-select picklist, checkbox, auto number to other types.
4:- Altering to multi-select picklist from any type except picklist.
5:- Changing to auto-number except from text .
6:- Changing from text-area to e-mail, phone, URL and text .