Effective August 9, 2018, USCIS updated policy regarding Unlawful Presence, which increases potential consequences for F-1/J-1 students who violate their status. The best way to avoid these consequences is to make sure you properly maintain your F-1/J-1 status while you are a student and during any F-1/J-1 post-completion training periods. Unlawful presence days begin accruing from the day an F-1 or J-1 violates status and can impact the ability to return to the US in the future, including 3 or 10 year bars on re-entry (dependent on number of days accrued). See here for BIO tips on How to Maintain your F-1/J-1 Student Status.
USCIS Issued a Final Policy Memo on August 9, 2018 outlining the changes and guidance which contains a number of exceptions and clarifications of when unlawful presence does NOT apply including: pre/post-completion grace periods, authorized reduced course load periods, annual vacations, change of educational level, OPT/OPT STEM and cap-gap periods, pending OPT/STEM applications, school transfers, during a timely field reinstatement application, and during a timely filed pending Change of Status application period. As of August 9, unlawful presence days DO begin accruing after F, J, or M nonimmigrant no longer pursues the course of study or the authorized program activity, or the day after he or she engages in an unauthorized activity; the day after completing the course of study or program (including any authorized practical training plus any authorized grace period; the day after the Form I-94 expires, if the F, J, or M nonimmigrant was admitted for a date certain; or the day after an immigration judge orders the student excluded, deported, or removed . For a detailed explanation of the changes, see NAFSA's ULP summary.