Ok - here's something that still works on a Summer 10 developer system, but isn't working today on a Winter 11 sandbox. I'm pretty sure it worked last week.

Steps to recreate/reproduce:
Create custom field named Test on the CampaignMember object. Data type string, any length.

Create an apex page to edit CampaignMember object (see VB code below). Any attempt to save this page will cause error:
Error: j_id0:cmform:j_id1:j_id5:j_id6: An error occurred when processing your submitted information.

Sample Apex page to reproduce:

<apex:page StandardController="CampaignMember" >
<apex:form id="cmform" >
<apex:pageBlock title="Campaign Member Edit" mode="edit">
<apex:pageBlockButtons >
<apex:commandButton action="{!save}" value="Save"/>
<apex:commandButton action="{!cancel}" value="Cancel"/>
</apex:pageBlockButtons>
<apex:pageblockSection columns="2" >
<apex:inputField rendered="true" value="{!CampaignMember.Test__c }"/>
</apex:pageblockSection>
</apex:pageBlock>
</apex:form>
</apex:page>

I'm pretty sure there are no security issues here - everything to repro was done under a system admin account. I've done a lot of experimenting with different fields - some standard fields cause the error, some don't.

So this leaves us with:

1. Any suggestions?

2. I would love to hear if anyone else can reproduce this on a Winter 11 instance.