This error caught me out. I am putting this post here firstly to remind me if I do it again and also to help others who may hit the same issue.

I also have been looking at Pester which is a framework for running unit tests within PowerShell

You will find some good blog posts about starting with Pester here

So I created a function script file Create-HyperVFromBase.ps1 and a tests script file Create-HyperVFromBase.tests.ps1 as shown.

The tests contained this code

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 $here = Split-Path -Parent $MyInvocation.MyCommand.Path
$sut = (Split-Path -Leaf $MyInvocation.MyCommand.Path).Replace(".Tests.", ".")
. {'$here\$sut'}
Describe "Create Hyper V from Base Tests" {
    Context "Parameter Values,Validations and Errors" {
        It exists {
        test-path function:\create-hypervmfrombase | should be $true
        } 
}
}

When I ran the test I got the following error

or

Googling pester “The script failed due to call depth overflow.” returned only 7 results but the Reddit link contained the information I needed

.Replace() is case sensitive. It didn’t remove the .tests. keyword from your file name. So it calls your test script again and repeats the same mistake over and over.

and so I renamed the tests script file to Create-HyperVFromBase.Tests.ps1 With a Capital T! and bingo

Don’t forget to name your Pester Tests scripts with a capital T when loading the script in this way and remember that Replace() is case sensitive.

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$here = Split-Path -Parent $MyInvocation.MyCommand.Path
$sut = (Split-Path -Leaf $MyInvocation.MyCommand.Path).Replace(".Tests.", ".")
. "$here\$sut"