If the storage system you are using has thin provisioning built in, a Primary VM can crash when it encounters an out of space error.

When used with a thin provisioned storage system, a Primary VM can crash. The Secondary VM replaces the Primary VM, but the error message "There is no more space for virtual disk <disk_name>" appears on the vSphere client.

If thin provisioning is built into the storage system, it is not possible for ESX/ESXi hosts to know if enough disk space has been allocated for a pair of fault tolerant virtual machines. If the Primary VM asks for extra disk space but there is no space left on the storage, the primary VM crashes.

The error message gives you the choice of continuing the session by clicking "Retry" or clicking "Cancel" to terminate the session. Ensure that there is sufficient disk space for the fault tolerant virtual machine pair and click "Retry".