For users of software libraries or public programming interfaces (APIs), backward compatibility is a desirable trait. Without compatibility, library users will face increased risk and cost when upgrading their dependencies. In this study, we investigate semantic versioning, a versioning scheme which provides strict rules on major versus minor and patch releases. We analyze seven years of library release history in Maven Central, and contrast version identifiers with actual incompatibilities. We find that around one third of all releases introduce at least one breaking change, and that this figure is the same for minor and major releases, indicating that version numbers do not provide developers with information in stability of interfaces. Additionally, we find that the adherence to semantic versioning principles has only marginally increased over time. We also investigate the use of deprecation tags and find out that methods get deleted without applying deprecated tags, and methods with deprecated tags are never deleted. We conclude the paper by arguing that the adherence to semantic versioning principles should increase because it provides users of an interface with a way to determine the amount of rework that is expected when upgrading to a new version.