According to Brian E. Fox there is something wrong with the Maven Core which causes the maven-dependency-plugin to fail if it is called by the mvn-eclipse-plugin. I can't tell any specifics since Brian looked into it, and only provided me a workaround. I'm pasting our dialog from the mailing list in here. Any further questions regarding this should be directed to Brian, since I am just a user and do not have the necessary insight.
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No, this is a maven core bug and will probably have to be fixed in 2.1, but file an issue anyway.
From: Michael Heß email@example.com
Sent: Thursday, February 14, 2008 12:57 AM
To: Maven Users List
Subject: RE: dependency:unpack vs. eclipse:eclipse
Thanks Brian, for finding this out.
I have created a workaround as suggested. Only additional thing I had to
do, was to also bind the resources:resources to process-resources phase,
because otherwise the filtering occured before the dependency:unpack. It's
dirty, but at least it works now.
Have you already taken care of filing a bug? If not, I would take care of
this. The bug is in the dependency-plugin, right?
Brian E. Fox schrieb:
> I am able to reproduce this and it's an unfortunate bug in 2.0.x. The only
> workaround I can suggest is to change the dependency plugin binding to a
> later phase than is invoked by the eclipse plugin. According to  the
> phase is generate-resources so you can bump it to process-resources.
> From: Michael Heß firstname.lastname@example.org
> Sent: Wednesday, February 13, 2008 1:07 AM
> To: Maven Users List
> Subject: RE: dependency:unpack vs. eclipse:eclipse
> here you go, I hope it somehow survives the transfer to the list. If it's
> completely garbled I can also send you the file directly as an attachment.
> Furthermore I'd like to add the error I'm getting when binding the
> dependendy-plugin unpack goal to a specific phase:
> ---- ERROR -----
> [INFO] One or more required plugin parameters are invalid/missing for
>  Inside the definition for plugin 'maven-dependency-plugin' specify the
> ---- ERROR -----
> But as you can see in the pom below, I do have the wanted configuration
> settings. Thanks for looking into this.
> bye, Michael
pom starts here--
> <?xml version="1.0" encoding="UTF-8"?>
> <project xmlns="http://maven.apache.org/POM/4.0.0"
> <build />
> <!--other profiles here as well - intentionally left them out -->
> <!-- copies the contents of "conf" to "target/conf" -->
> <!-- Java 1.5 -->
> <!-- if I put this in, no more eclipse:eclipse
> Brian E. Fox schrieb:
>> The faq entry about the executions/non executions would only apply when
>> running dependency:unpack directly...it shouldn't have any issues with
>> eclipse:eclipse. (although I haven't tested it recently).
>> Can you paste your pom so I can look at it?
>> From: Michael Heß email@example.com
>> Sent: Tuesday, February 12, 2008 10:34 AM
>> To: firstname.lastname@example.org
>> Subject: dependency:unpack vs. eclipse:eclipse
>> Hi list,
>> I've been searching and fiddling around all day, with no success so far.
>> So this is my only hope of finding an answer.
>> My basic setup looks like the following:
>> I have an artifact which creates a ZIP-assembly. Lets call it
>> Furthermore I have anotherArtifact which depends on
>> artifact-resources.zip. As the name states, the artifact does not
>> any sources, but resources. These resources I want to have unpacked and
>> filtered during anotherArtifacts build. So what I did, was to add a
>> dependency-plugin-configuration to anotherArtifacts pom.xml, and I bound
>> it to the generate-resources phase. At that point everything was still
>> fine. My build went all the way through, the resources got unpacked and
>> filtered, and were used correctly afterwards as the build lifecycle went
>> Then I wanted to use maven-eclipse-plugin on anotherArtifact. At that
>> point, I ran into the problem, that dependency:unpack fails, because
>> eclipse:eclipse seems to trigger it in the "CLI" way. Although I do not
>> really know what it maven internally means, I did as I was told in the
>> maven-dependency-plugin faq, and removed my execution entries out of the
>> configuration. This fixed the eclipse:eclipse problem.
>> But, now this again leads to the following problems:
>> Since my maven-dependency-plugin configuration is no longer bound to any
>> specific phase it should run during process-sources. At least the
>> documentation says so. But it does not, I don't know why.
>> So, right now my conclusion for today is (and I really do hope someone
>> prove me wrong):
>> Either - I can have dependency:unpack attached to a specific phase
>> OR - I can have eclipse:eclipse working
>> As stated before. I spent the whole afternoon trying things out.
>> I probably learned a lot, I'm desperate for help. I really do not have
>> more ideas what to do.
>> Thanks for reading all this, and maybe providing any insight.
>> bye, Michael