No it is not amateur at all. How is slipping ONE OR TWO WEEKS because we found an undressed bug is being "amateur" ?
Amateur would be if we keep slipping with no end in sight "one more week, one more week, one more month" or if we ignored the bug and said "but but the deadline is in 2 days we have to ship on time".
That being said yeah it does make sense to try to avoid that "last minute bugs" but when they happen (and they will sometimes) it isn't the end of the world like some are viewing it.