We had the following. This happened only in IE (Firefox coped fine), when a LinkButton was clicked in a .NET page.
Invalid postback or callback argument. Event validation is enabled using
If this happens to you, the first thing you should check is simple.
Do you have more than one form in the page? Is one form nested inside the other?
That's what we had (DOH) - taking out the inner form, and all will be hunky dory.
Not that two forms is valid anyway - it was just a regular joe c*ck up - but nonetheless, I thought it was particularly unhelpful of .NET to spew out an irrelevant error!