You could receive a counter offer from your present employer and there have been many studies done on that particular question. The studies have shown that most people who accept a counter offer from their employer are typically terminated or resign within the following 6 months. There are a couple of reasons for this:
1. By accepting the counter offer you are now giving your employer time to find a replacement for you. Most companies do not want to keep someone on staff that was ready to leave them. The bond of trust is broken. Usually they are not prepared to lose you and need time to find a replacement. That is why they counteroffer you.
2. They also now feel that you are getting exactly what you wanted and not what they wanted to pay you. This could also force them to have to pay other employees more to stay in line with your new increase. From here on out every time you have a doctor's appointment, need to leave early or come in late you have your employer thinking you are interviewing even when it is a legitimate excuse.