Updates are currently disallowed on GET requests. To allow updates on a GET, set the 'AllowUnsafeUpdates' property on SPWeb.

So today I hit a limit on SharePoint when listing from a library

The attempted operation is prohibited because it exceeds the list view threshold enforced by the administrator

The solution is simple, just increase the number of items, so from the Central Administration Site:

1. Application Management -> Manage Web Application and select your web application
2. In the Ribbon, click on General Settings drop-down and choose “Resource Throttling”.
3. In the “List View Threshold”, increase the value

The problem I was having was that when I tried to do this I would get the following error:

Updates are currently disallowed on GET requests.  To allow updates on a GET, set the 'AllowUnsafeUpdates' property on SPWeb.

The solution, from the SharePoint PowerShell console:

$sp = get-spwebapplication https://myapp
$sp.HttpThrottleSettings
$sp.Update()

The problem seems to be related to the web application not having a value for HttpThrottleSettings, which will be set by running the above commands.