Under state and federal law, failing to appear in court after bailing out of jail is its own crime. That means that defendants who "jump bail":
Some states define bail jumping as a defendant failing to show up in court, thereby forfeiting bond, then failing to surrender within a set time period. Many provide that the defendant has 30 days to surrender after bond forfeiture. Because bail jumping is its own offense, the defendant's innocence as to the charges that led to arrest typically isn't a defense to the failure to appear.
Some states make bail jumping an offense only where the defendant faces a felony, while others provide that any kind of charge will suffice. Some have different degrees of bail jumping—for instance, a misdemeanor version when the underlying charges are misdemeanors and a felony version when they're felonies. (For more on not showing up for court, which can be called "bail jumping" even if the defendant has made the first court appearance after getting out of jail, see Failure to Appear in Court and What happens if I fail to show up in court after bailing out of jail?)
In most states, the prosecution must prove that the defendant intentionally failed to show up to court—it will usually suffice to show that the defendant knew of the requisite court appearance and simply didn't appear. In these states and others, there must be proof that the defendant received notice of the appearance date. Depending on the jurisdiction, that notice can take various forms, including a letter mailed to the defendant and even the wording of the bail bond.
Whether a defendant's excuse for not showing up to court (and potentially failing to surrender thereafter) constitutes a defense depends on the facts and the jurisdiction. It's often a valid defense that the defendant couldn't have avoided the failure to appear due to circumstances beyond his or her control. But courts have rejected excuses due to intoxication and drug use, and have been skeptical of poorly substantiated claims of illness. In some states, even the fact of being incarcerated in another jurisdiction isn't a cognizable excuse for failing to be in court when required.
Example: After her arrest for drug possession, Kelly posts $5,000 to bail out of jail. After signing a bail document that indicates the date for her court appearance and receiving separate notice of the date in the mail, she fails to show up for the appearance. Some 31 days later, she still hasn't turned herself in. After her re-arrest, at her trial for bail jumping, two witnesses testify that they saw her the day after her scheduled court appearance, and that she appeared to be really sick. A jury convicts her of bail jumping and she appeals. The court rejects her excuse, noting that she presented insufficient evidence that she couldn't have made it to court; she didn't present proof along the lines that she had been in the hospital when she was supposed to be in court. (State v. Fredrick, 123 Wash. App. 347 (2004).)
If you've been charged with bail jumping or simply missed your court date and don't know what to do, talk to a local criminal defense attorney. A knowledgeable lawyer will be able to advise you as to the applicable law, the local practices, and your best course of action, including whether surrendering to the court or the authorities may prevent a bail jumping charge.