An IllegalStateException is a runtime exception in Java that occurs when a method is invoked at an inappropriate time, similar to attempting to start a car while it's in "Drive." This exception can arise in situations involving threads or the Collections framework, such as calling Thread.start() on an already started thread, using Iterator.remove() without prior invocation of next(), or adding elements to a full Queue. To prevent IllegalStateException, developers should ensure that methods are called at appropriate times, such as using Iterator.next() before remove() to adjust the iterator's position, or employing Queue.offer() instead of add() to respect capacity limits. Managing errors like IllegalStateException is crucial for robust Java applications, and tools like Rollbar can automate error monitoring and triaging to enhance error management and code deployment confidence.