It really depends on a lot of factors:
What is the purpose of the antibiotics?
If it is meant for life-saving purposes like in a hospital, missing a dose of intravenous antibiotics could mean a death sentence for the patient.
Otherwise, if it is for sinusitis or urinary tract infections, or similar low severity infections, missing a dose will probably be rather inconsequential.
The best person to ask would be your GP.
What is the type of antibiotics?
Some antibiotics kill bacteria by having a high dose in your body, while others kill by maintaining a certain level in your body. If it is the second type, missing a dose might cause the dose of antibiotics to fall below the level with which it can work, and can encourage the bacteria to become resistant to the antibiotics.
How many times do you need to take it in a day?
Obviously if it is just one dose a day, it is probably very important. Whereas if it is 5 times a day, missing a single dose probably isn't going to be very inconsequential.
The TL,DR; is, don't forget.
And if you do forget, try to take it back as soon as possible, and do not double dose.
