The Java error "Could not find or load main class" occurs when the Java Virtual Machine (JVM) cannot locate or load the main class during program execution, often due to simple mistakes such as incorrect class names, mismatched directory structures, or misconfigured classpaths. This error typically arises in command-line executions and can be caused by various issues such as declaring the class in the wrong package, file path mismatches, typos, or missing dependencies. For instance, incorrectly specifying "Myclass" instead of "MyClass" can trigger this error. Solutions include verifying the correct class name without the .class extension, ensuring the correct file path, and properly defining the classpath. Tools like Rollbar can aid in tracking and managing such errors by automating error monitoring and triaging, thereby facilitating easier error resolution.