Throttling can be enabled and disabled on a list-by-list basis to overcome some temporary threshold issues.
Check if throttling is currently enabled:
$web = Get-SPWeb http://URL_OF_YOUR_SITE
$list = $web.Lists["NAME_OF_YOUR_LIST"]
This will return true or false according to the Throttling settings. If it returns True, the the List has been throttled (has more items than the List View Throttle limit). If it returns false, it has either less than the List View Throttle limit or throttling has been disabled via the EnableThrottling property.
To enable throttling on a list, simply run the following