Missing Tags – This is the most common error of wrong data. This generally happens when new pages are added or the exiting pages are redesigned/recoded and the developer forgets to include the tags. Make sure all of your pages are tagged with Google Analytics code. You can use a tool like GAChecker, to verify if the Google Analytics tags are missing on any pages of your site.
Mistagged Pages – Incorrect implementation or double tagging leads to wrong data in Google Analytics. Double tagging results in increased page views and a low bounce rate. If you bounce rate is lower than 20% then that’s the first thing you should check.
Incorrect Filters – Wrong Filters can mess up the data and distort the view. Always create an unfiltered view so that you have correct data to fall back on.
Sampling – Sampling happens on highly trafficked site. Sampling in Google Analytics is the practice of selecting a subset of data from your traffic and reporting on the trends available in that sample set. For most purposes, this might not be a non-issue however it can be of concern in eCommerce sites where sampling can (will) result in wrong sales figures. You can get more information about GA sampling on “How Sampling Works“.