[System.Windows.LayoutCycleException] = {System.Windows.LayoutCycleException: Layout cycle detected. Layout could not complete.}

Having code like this:

<phone:LongListSelector>
  <phone:LongListSelector.ItemTemplate>
    <DataTemplate>
      <StackPanel>
        <Image/>
      </StackPanel>
    </DataTemplate>
  </phone:LongListSelector.ItemTemplate>
</phone:LongListSelector>

and a problem like this:

[System.Windows.LayoutCycleException] = {System.Windows.LayoutCycleException: Layout cycle detected.  Layout could not complete.}

you should do this:

<StackPanel Width="446">

or this:

<Image Stretch="None"/>

Problem solved.