It's strange that CAML query can only compare a pair of expressions either by <And> or <Or>. So if you have a query like the one follows, you will see an error message "Cannot complete this action. Please try again", which is not very helpful.

<Where>
  <And>
    <Gt>
        <FieldRef Name='Created' /><Value Type='DateTime'><Today OffsetDays='-30' /></Value>
    </Gt>
    <Neq>
        <FieldRef Name='ContentType' /><Value Type='Text'>Folder</Value>
    </Neq>
    <Neq>
        <FieldRef Name='ContentType' /><Value Type='Text'>InfoPath Form Template</Value>
    </Neq>
  </And>
</Where>

The workaround is to use nested <And> or <Or>, in which each operator only takes two expressions. The following snippet corrects the above one. This will make things more complicate especially for a number of conditions  mixed with <And> and <Or>.

<Where>
 <And>
  <And>
    <Gt>
        <FieldRef Name='Created' /><Value Type='DateTime'><Today OffsetDays='-30' /></Value>
    </Gt>
    <Neq>
        <FieldRef Name='ContentType' /><Value Type='Text'>Folder</Value>
    </Neq>
   </And>
    <Neq>
        <FieldRef Name='ContentType' /><Value Type='Text'>InfoPath Form Template</Value>
    </Neq>
  </And>
</Where>