javac error but build SUCCESS

Please attach a sample project to reproduce this issue.

Sorry, that is not so easy, because I don't get any error. So I don't know the reason why it is failing. Can I provided more information? should I debug it? Can you point me to sourcecode where to debug?

Here you can find examples of integration tests. These are just mini-project to reproduce it (you said you can reproduce it always). That would be a start. Next we'll have to write a verify-script. Your expectation is "generated classes", which is something we can check.
Right now I have no idea what your project looks like (folder-structure, files), how you configured the m-compiler-p, so it's impossible to reproduce it.

Unfortunately I've run into this one as well. Not sure what the problem is. The project with the problem includes code in src/main/java as well as generated source code.
@Jochen: Is that similar to your problematic project?

Due to MCOMPILER-81 it's very hard to find the root cause. After creating a patched plexus-compiler-javac so that I would get all debug info, I see that the problem is that the following message from the compiler isn't catched as an error:
"An exception has occurred in the compiler (1.6.0_24). Please file a bug at..."

Just sent a pull request on plexus-compiler (https://github.com/sonatype/plexus-compiler/pull/7) that contains a fix of the existing band-aid catching mechanism of unparseable errors. This ensure that the scenario described in this ticket shouldn't happen I believe (at least it solves the scenario I ran into).
plexus-compiler-javac should handle this better (logging wise) but this will at least prevent m-compiler-p from incorrectly reporting compilation as successful.

Could someone with commit rights for plexus-compiler take a look please?

For the record, I can't figure out how to write an integration test for this one as the root cause is an error message which can't be parsed correctly (for example an exception/bug in the compiler) in combination with some warning messages which are parsed correctly.
Don't know how to force an exception in the compiler which would be compiler and compiler version neutral (as well as platform neutral).

Waiting for a new release of plexus-compiler.

The rework in the upcoming plexus-compiler:2.0 should solve this as there is now a separate success flag returned which is based on the actual return code from the compiler.

@Jochen: Could you please test 3.0-SNAPSHOT of maven-compiler-plugin on a problematic project?

I'm sorry to say, that I don't have access to that source code anymore and so I'm not able to test this new version. Thank's a lot for your work anyway! I'm sure that will help in the future.

Closing as fixed based on code analysis. Unfortunately I'm not able to verify a Snapshot version on the project I've seen a similar issue.

Just to report back that m-compiler-p 3.0 now correctly fails my problematic build and prints "An exception has occurred in the compiler (1.6.0_24). Please file a bug at..." to the console.