Unable to Verify $_POST Vars Error

If you are getting an error like this after a purchase has been made...

ERROR: Unable to verify $_POST vars.

There are two main things to try to fix this error:

PayPal PDT

If your Paypal PDT (Payment Data Transfer) information is missing or incorrect, this can trigger this error. Please verify that your PDT configuration is correct. 

To do this, go to OptimizeMember -› PayPal® Options -› PayPal® PDT Integration
Server Scanner
This script will help spot some known server issues that would cause you this error -  OptimizeMember Server Scanner
Follow the instructions in that article to spot these issues, and follow the advice given to solve each until you get an all-green.