An error occurred while enumerating through a collection: Collection was modified; enumeration operation may not execute...

Below code will generate above error:

$someHashTable = @{}
$someHashTable.Add("key1", "value1")
$someHashTable.Add("key2", "value2")
$someHashTable.Keys | %{ $someHashTable[$_] = "newValue" }

It should looks like below code - collection of keys must be copied to fresh new collection.

$someHashTable = @{}
$someHashTable.Add("key1", "value1")
$someHashTable.Add("key2", "value2")
$keys = @(); $someHashTable.Keys | %{ $keys += $_ }
$keys | %{ $someHashTable[$_] = "newValue" }