Google Analytics just recently added a means of notifying you of problems so you didn't have to change anything to cause this issue. It could be preexisting.
EDIT: Solution 1 and 2 may help, but what worked was "Solution 3" -- Upgrade to Universal Analytics and increase session length.
Solution 1 - Send less tracking data
If the Google Analytics property is for a high traffic site without Google Analytics Premium you are limited to 10 million hits per month including events Data Limits - Analytics Help. In this case analytics may have recorded a user's event but the pageview was ignored to keep your account within limits.
If you could be flirting with this limit try sending less event tracking by reinventing or removing some event tracking entirely. If you are blatantly violating the data limits track only a subset of users by setting the Sample Rate in your analytics.js (or ga.js) code.
Solution 2 - Find the problematic event code
If you are not going over the data limits you have an event being sent before a pageview.
Methods of debugging "(not set) entry present in reports for property" are described on this page What the value (not set) means - Analytics Help.
Verifying this issue:
Go to the Analytics property with the issue and click Behavior reports > Site Content > Landing Pages
If you see landing page = (not set), this is generally due to a session with no page or screen view level hits included.
It’s possible to have a session that doesn’t include a page or a screen view, but does include another kind of interaction hit type, like an Events or Ecommerce hit type.
Debugging this issue:
Identify which hit type is causing the issue:
use the advanced filter (found at the top of the data table) to
restrict the data to include page/screen views matching exactly 0
For each of these 3 reports:
- Behavior > Events
- Acquisition > Social > Plugins
- Conversions > Ecommerce reports
Note: The "page/screen views" dimension may have disappeared in advanced filters under those reports, I will update this answer when I find out more.
Solution 3 - Session is expiring before user leaves site
You can configure a visitor's session length for your property in Session Settings under Tracking Info.
The default length of time for a session or campaign. An individual session or campaign for a given user ends after the amount of time specified here has passed (counting from the start of the session or campaign), so long as the session or campaign has not been stopped though another means. Learn more about Learn more about session and campaign timeout handling.
If the user's session expires but they haven't left the page and another event triggers there will be no page view before the event has triggered. In this case your site would fall under this category:
The length of a session and campaign depends on your site and business. Here are a few ideas to get you started thinking about session and campaign timeouts:
Lengthen the session time if you have a lot of content and expect users to take a long time engaging with that content. Conversely, shorten the session time if the site has a small amount of content. 5
Increase the session length under Session Settings so these events will still have an associated pageview.
Note: You can only configure the session length if you're using Universal Analytics.