I was recently offered a job out of graduate school and did not expect any issues with my background check. Unfortunately, a discrepancy for employment end date showed up for my undergraduate research job, which officially started in June and ended in August of the same year (paid), but I continued the work until May the following year for credit instead of pay. I ended up listing it as June to May of the following year but the check came back as June to August of the same year.
This was completely unexpected and now I'm worried it looks like a lie, especially since I only saw this AFTER the background check was completed.
EDIT: I emailed my hiring manager and HR about the discrepancy and they seem to find it insignificant and may or may not contact my reference, who I informed of the issue as well. It looks like I still have the job and the starting date has not changed.