Having a list in your controller that is displayed as input fields either through pageblock or repeat and you need to add another row to the list via a button.  Chances are when you enter data on any input field on any row and you hit Add Row button that has a rerender attribute that makes the ajax call, the previous entered data will be lost as per code sample below.

public class MyController {

public list<myOject__c> objList {get;set;}

public MyController() {

objList = new List<myObject__c>();

}

public void addRow(){

myObject__c newObj = new myObject__c();

objList.add(newObj);

}

}

VF:

<apex:pageblock id=”myList”>

<apex:pageblockSection value=”{!objList}” var=”obj”>

<apex:column>

<apex:inputField value=”{!obj.Name” />

</apex:column>

</apex:pageBlockSection>

<apex:commandButton value=”Add Row” action=”{!addRow}” rerender=”myList”/>

</apex:pageblock>