and what are the consequences if I did?
US visas are solely for entry, so whether you have a valid visa or not is irrelevant for your stay inside the US.
F1 regulations require you to be in good standing in your program of study with a valid I-20, and you get to remain in F1 status for 60 days after you successfully complete your program. So even assuming that you completed your program at your I-20 expiration date, the latest you could have been in status would have been 60 days after that. So you must have been out of status for at least a month.
There are no statutory consequences for having been out of status per se. You could have been deported during the period you were out of status, but I am assuming that that didn't happen. There is an increased chance that you will be denied nonimmigrant visas or denied entry as a nonimmigrant in the future due to "failure to overcome the presumption of immigrant intent". This is a subjective determination that is up to the officer.
You did not accrue "unlawful presence". As an F1 student, you are admitted as "D/S" on your I-94, so you do not automatically start accruing unlawful presence no matter how long you stay. You would only start accruing unlawful presence if you applied for some benefit to USCIS and were denied, or you were put into removal proceedings and ordered removed. Even if you had accrued 1 month of unlawful presence, it would not have any statutory consequences, since you would only trigger a ban if you leave after accruing more than 180 days of unlawful presence.