What if, for some random reason you wanted to compare two SharePoint Lists or Libraries and find the differences.

For the setup, I created two Lists (A and B).  I then added a few items to the Lists. Notice, that List A has two extra items, 6 and 7.

     

 

$mSite = Get-SPweb "http://sharepointed.com/site/taco"
$aList = $mSite.lists["A"]
$bList = $mSite.lists["B"]

$arrA = @()
$arrB = @()

foreach($iA in $aList.Items)
{
 $arrA += $iA["Title"]
}

foreach($iB in $bList.Items)
{
 $arrB += $iB["Title"]
}

$c = Compare-Object -ReferenceObject $arrA -DifferenceObject $arrB -PassThru
Write-Host $c

Output of the above script is: 6 7

More about the Compare-Object cmdlet:

http://technet.microsoft.com/en-us/library/ee156812.aspx