This happens in AS7 pulled from GIT on 2011-04-12.

1) Download this sample app: http://www.cs.hs-rm.de/~knauf/KomponentenArchitekturen2008/stateless/StatelessNoAnnotation.ear
2) Rename to "Stateless.ear"
3) Deploy it to "standalone\deployments". This will result in a "Stateless.ear.failed" file.
4) Be nasty and add a file "Stateless.ear.isdeployed". Now you will see this exception in the server console:

21:13:13,828 ERROR [org.jboss.as.deployment] (DeplooymentScanner-threads - 2) Scan of C:\temp\jboss-7.0.0.Beta3-SNAPSHOT\standalone\deployments threw Exception:
java.lang.NullPointerException
at org.jboss.as.server.deployment.scanner.FileSystemDeploymentService$DeploymentMarker.access$1000(FileSystemDeploymentService.java:1008)
at org.jboss.as.server.deployment.scanner.FileSystemDeploymentService.scanDirectory(FileSystemDeploymentService.java:415)
at org.jboss.as.server.deployment.scanner.FileSystemDeploymentService.scan(FileSystemDeploymentService.java:288)
at org.jboss.as.server.deployment.scanner.FileSystemDeploymentService$1.run(FileSystemDeploymentService.java:134)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:441) [:1.6.0_24]
at java.util.concurrent.FutureTask$Sync.innerRunAndReset(FutureTask.java:317) [:1.6.0_24]
at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:150) [:1.6.0_24]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$101(ScheduledThreadPoolExecutor.java:98) [:1.6.0_24]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.runPeriodic(ScheduledThreadPoolExecutor.java:180) [:1.6.0_24]
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:204) [:1.6.0_24]
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886) [:1.6.0_24]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908) [:1.6.0_24]
at java.lang.Thread.run(Thread.java:662) [:1.6.0_24]
at org.jboss.threads.JBossThread.run(JBossThread.java:122)

Though this should not really when a user knows what to do, it is at least an "outside event" which should not "confuse" the deployment services.