Reasons for Invalid Data entries in Data Validation drop down:

Although you have created data validation dropdown arrows on some cells, users may be able to type invalid entries. The following are the most common reasons for this.
Error Alert
If the Error Alert is turned off, users will be able to type any entry, without receiving an error message. To turn the alert on:
1. Select the cell that contains a data validation list
2. Choose Data|Validation
3. On the Error Alert tab, add a check mark to the Show error alert after invalid data is entered box.
4. Click OK


Blank Cells in Source List
If the source list is a named range that contains blank cells, users may be able to type any entry, without receiving an error message. To turn prevent this:
1. Select the cell that contains a data validation list
2. Choose Data|Validation
3. On the Settings tab, remove the check mark from the Ignore blank box.
4. Click OK

Note: If the source list contains blank cells, and is a range address, e.g. $A$1:$A$10, it will block invalid entries with Ignore blank on or off.