Azure AD/Entra ID (Microsoft IDP) was most common and amount of IT folks who don't have a clue about it is staggering.
Companies kicking over issues to us when it's their problem. "Hey, we have a ticket saying MFA Required but account shows as Entra ID." "Send it back with contact their IT team." "Their IT team opened the ticket" rage screaming
Companies not following setup instructions. I used to provide Terraform, Powershell and Graphical setup. I can count on one hand how many people used Terraform/Powershell. This was always dicey because I got familiar with the error messages and would be "Yep, this was not setup right on their end." I had 4 phone calls with $CustomerIT swearing it was setup properly and stopped attending after that. Finally they got someone with a brain to review and finished setup.
Documentation would fall out of date because of some UI change and I'd spend a day reviewing it and updating it.