It can bring serious consequences, such as being deemed ineligible for a new visa in the future.
The I-94 form, which you can find online here, states the number of days you can stay in the United States after being admitted. If you entered using a B visa, your period of stay will most likely be somewhere between 90 days and six months, but it depends solely on the Port of Entry officer that allows you into the United States.
If you entered the U.S. with a student visa (F, M or J visa), your I-94 will show the code D/S, which means that you have permission to stay in the country for the duration of your student status.
Keep in mind: Your visa’s expiration date and the length of time you may stay in the United States are two different things, For more information, click here.