$anonymous = Get-User -Identity "extranet\Anonymous"
$testItem = Get-Item -Path master:\Content\Home
New-UsingBlock (New-Object Sitecore.Security.Accounts.UserSwitcher $anonymous) {
$testItem.Title = "If you can see this title it means that anonymous users can change this item!"
New-UsingBlock : Exception setting "Title": "Exception calling "Modify" with "3" argument(s): "The current user does not have write access to this item. User: extranet\Anonymous, Item: Home ({110D559F-DEA5-42EA-9C1C-8A5DF7E70EF9})""
+ New-UsingBlock (New-Object Sitecore.Security.Accounts.UserSwitcher $a ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [New-UsingBlock], SetValueInvocationException
+ FullyQualifiedErrorId : ScriptSetValueRuntimeException,Spe.Commands.Data.NewUsingBlockCommand