Start here: https://wyday.com/limelm/help/faq/#useful-reports
Version numbers for everything. Also, product keys, etc.
Also, ensure the customer was not activated on a virtual machine to begin with (i.e. you didn't disable VM activations after they already activated on a VM).
We can't reproduce this behavior on real machines. We *can* reproduce this on VMs (which is why your should *always* disable VM activations and use TurboFloat).