Are you in Find Mode when you do this? If not, you should be. Otherwise you're changing the value in the field with the drop down in the record you're on. It sounds like you should disable entry to this field in Browse Mode.
I'd say Rick's guess is most likely the issue. But a similar issue can arise when you are either creating a new record linked to projects or have a portal to the related table and you want selecting a value to control what records appear in a portal. In those situations, if selecting a value is modifying data, the most likely issue is that you have the wrong field set up with the drop down list format.
So please describe your layout and relationships in much more detail if Our two posts isn't enough to get things working correctly.
Hi I am having the same issue where I have a drop down on my layout and depending on the value that is chose the portal shows records. The portal works fine but the drop down value - the last value keeps getting overwritten with the value that is chosen in the drop down. Are there any examples of portals showing records based on a drop down value on the forums? Please help.