Most of us have encounted this error when working with JSP pages. If JSP pages are compiled on the fly, then it is possible that errors are present in it that cannot be resolved at runtime. Hence it is a good idea to compile your JSPs as U develop them. Most of the modern IDE's have support for compiling JSP's during the 'build' process.
Recently I encountered the same error while building a web application on Netbeans. I was getting the "java.lang.error - Unresolved compilation problem" for a Struts Action class, that was referencing another bean. It was strange that this error was not caught by the compiler. I did a 'clean build' again and everything was OK. But still at runtime, I got the same error.
I was perplexed and could not understand what was the problem.
Then suddenly inbetween, I started getting ClassCastExceptions. It was then I reliazed that there was older classes in the classpath somewhere, that was messing things up.
Closer inspection revealed that someone had placed "classfiles" into source-control (VSS) and when we did 'get-latest', the old class files were also downloaded into our directory.
Moral of the story: Whenever U get ClassCastExceptions or Unresolved Compilation problems at runtime, check if U have stale class files somewhere in the classpath.