The ubiquitous use of third-party libraries in software development has enabled developers to quickly add new functionality to their client software. Unfortunately, library usage also carries a cost in terms of software maintenance: library upgrades may include breaking changes, in which client expectations about library behaviour are no longer met in new library versions. Behavioural breaking changes can be particularly insidious, and in their full generality, could require sophisticated program analysis techniques to (approximately) detect. In this work, we present our UnCheckGuard tool, which detects a class of behavioural breaking changes—those related to exceptions thrown by Java libraries. UnCheckGuard analyzes both sides of the library/client duet. On the library side, UnCheckGuard creates a list of new exceptions that may be thrown by methods in a library’s public API, including by its transitive callees. On the client side, UnCheckGuard identifies client methods that call library methods with new exceptions. To reduce false positives, UnCheckGuard additionally filters out new exceptions that cannot be triggered by particular clients, using taint analysis. It therefore can be used by client developers as a tool to screen library updates for relevant incompatibilities. We have evaluated UnCheckGuard on 302 libraries and 352 library-client pairs drawn from the DUETS collection and found 120 libraries with newly-added exceptions, as well as 1708 callsites to library methods which, when upgraded to the latest version, may introduce a behavioural breaking change in the client due to a newly added unchecked exception. These findings highlight the practical value of UnCheckGuard in identifying exception-related incompatibilities introduced by library upgrades.