When a external tool re-wrote our poms, it added a xml header:
<?xml version="1.0" encoding="UTF-8"?>

before the <project> tag.

This made maven-plugin blow up:

ERROR: Failed to parse POMs
java.io.IOException: remote file operation failed: /XXX at hudson.remoting.Channel@1fabae96:build-03: hudson.remoting.ProxyException: hudson.maven.MavenModuleSetBuild$MavenExecutionException: org.apache.maven.project.ProjectBuildingException: Some problems were encountered while processing the POMs:
[FATAL] Non-parseable POM /XXX/pom.xml: expected start tag name and not < (position: START_DOCUMENT seen <<... @1:2) @ line 1, column 2

at hudson.FilePath.act(FilePath.java:987)
at hudson.FilePath.act(FilePath.java:969)
at hudson.maven.MavenModuleSetBuild$MavenModuleSetBuildExecution.parsePoms(MavenModuleSetBuild.java:960)
at hudson.maven.MavenModuleSetBuild$MavenModuleSetBuildExecution.doRun(MavenModuleSetBuild.java:679)
at hudson.model.AbstractBuild$AbstractBuildExecution.run(AbstractBuild.java:537)
at hudson.model.Run.execute(Run.java:1741)
at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:531)
at hudson.model.ResourceController.execute(ResourceController.java:98)
at hudson.model.Executor.run(Executor.java:408)
Caused by: hudson.remoting.ProxyException: hudson.maven.MavenModuleSetBuild$MavenExecutionException: org.apache.maven.project.ProjectBuildingException: Some problems were encountered while processing the POMs:
[FATAL] Non-parseable POM /XXX/pom.xml: expected start tag name and not < (position: START_DOCUMENT seen <<... @1:2) @ line 1, column 2

at hudson.maven.MavenModuleSetBuild$PomParser.invoke(MavenModuleSetBuild.java:1364)
at hudson.maven.MavenModuleSetBuild$PomParser.invoke(MavenModuleSetBuild.java:1098)
at hudson.FilePath$FileCallableWrapper.call(FilePath.java:2691)
at hudson.remoting.UserRequest.perform(UserRequest.java:121)
at hudson.remoting.UserRequest.perform(UserRequest.java:49)
at hudson.remoting.Request$2.run(Request.java:326)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:68)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
at ......remote call to build-03(Native Method)
at hudson.remoting.Channel.attachCallSiteStackTrace(Channel.java:1413)
at hudson.remoting.UserResponse.retrieve(UserRequest.java:221)
at hudson.remoting.Channel.call(Channel.java:778)
at hudson.FilePath.act(FilePath.java:980)
... 8 more
Caused by: hudson.remoting.ProxyException: org.apache.maven.project.ProjectBuildingException: Some problems were encountered while processing the POMs:
[FATAL] Non-parseable POM /XXX/pom.xml: expected start tag name and not < (position: START_DOCUMENT seen <<... @1:2) @ line 1, column 2

at org.apache.maven.project.DefaultProjectBuilder.build(DefaultProjectBuilder.java:364)
at hudson.maven.MavenEmbedder.buildProjects(MavenEmbedder.java:361)
at hudson.maven.MavenEmbedder.readProjects(MavenEmbedder.java:331)
at hudson.maven.MavenModuleSetBuild$PomParser.invoke(MavenModuleSetBuild.java:1301)
at hudson.maven.MavenModuleSetBuild$PomParser.invoke(MavenModuleSetBuild.java:1098)
at hudson.FilePath$FileCallableWrapper.call(FilePath.java:2691)
at hudson.remoting.UserRequest.perform(UserRequest.java:121)
at hudson.remoting.UserRequest.perform(UserRequest.java:49)
at hudson.remoting.Request$2.run(Request.java:326)
at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:68)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)

I would have expected maven-plugin to handle that.