Beginning with SharePoint 2010, Microsoft introduced a “Threshold Limit” per list. By default, this is set to 20,000 for administrators and 5,000 for contribute users.
If your Flex site is coming close to the default there may be unusual results when users attempt to perform functions. Always check this setting and include the details when writing to support@lookout-software.com.
An example problem that may occur would be a list containing 25,000 items which a user is attempting to return 100 items from it (as in a view). Although that would normally not cause the threshold to be reached, if that view also sorts on a column it may require all 25,000 items to be scanned first before the 100 items are determined and in this case, the threshold would be hit.
The most direct way around these issues is to raise the threshold limit in Central Administration. Many of our clients have done this and have experienced no adverse outcomes after doing so.
This will affect all lists in the specified web application.
More information on Threshold Limits can be found here: https://msdn.microsoft.com/en-gb/library/ff798465.aspx