Running tests using dynamic attach (com.sun.tools.attach API) I found that on linux attempt to attach intermittently fails with 2 kind of exceptions:

java.io.IOException: well-known file is not secure
        at sun.tools.attach.LinuxVirtualMachine.checkPermissions(Native Method)
        at sun.tools.attach.LinuxVirtualMachine.<init>(LinuxVirtualMachine.java:111)
        at sun.tools.attach.LinuxAttachProvider.attachVirtualMachine(LinuxAttachProvider.java:63)
        at com.sun.tools.attach.VirtualMachine.attach(VirtualMachine.java:213)

java.io.IOException: Connection refused
        at sun.tools.attach.LinuxVirtualMachine.connect(Native Method)
        at sun.tools.attach.LinuxVirtualMachine.<init>(LinuxVirtualMachine.java:118)
        at sun.tools.attach.LinuxAttachProvider.attachVirtualMachine(LinuxAttachProvider.java:63)
        at com.sun.tools.attach.VirtualMachine.attach(VirtualMachine.java:213)

To reproduce extract attached archive and run script test_attach.sh, first script parameter - tested JDK. This script in loop executes simple test using dynamic attach. Test fails very intermittently (~ 1 of 500), but since we have many tests using dynamic attach possibility to face such failures is pretty high.

Failure reproduces for example on vmsqe-p4-08.russia (SuSe 10), vmsqe-xeon-02.russia (RedHat 4).
This failure mode has been seen in nightly testing. Here is the
entry from my analysis report:

New MM_REGRESSION failures (from 2008.02.28)
* sun/management/jmxremote/bootstrap/LocalManagementTest.sh
        This test failed due to "IOException: well-known file is not
        secure" on Linux IA32 Server VM (machine jtg-linux17).

        Update: This failure might be related to the following:
                6649594 3/4 Intermittent IOExceptions during dynamic
                            attach on linux

            I'm checking with Alan.
This failure mode has been seen in nightly testing. Here is the
entry from my analysis report:

New nsk.jvmti failures (from 2008.09.18)
    nsk/jvmti/AttachOnDemand/attach039
        This test failed due to "IOException: well-known file is not
        secure" on Linux IA32 Server VM (machine tq-win2k-exe).
Another sighting from nightly testing:

New nsk.jvmti failures (from 2008.09.26)
    nsk/jvmti/AttachOnDemand/attach015
        This test failed due to "IOException: well-known file is not
        secure" on Linux AMD64 Server VM (machine wowamd). This is
        an occurrence of the following bug:

            6649594 3/4 Intermittent IOExceptions during dynamic attach on linux

        I will copy this entry to 6649594.
Another sighting from nightly testing:

New nsk.jvmti failures (from 2008.09.30)
* nsk/jvmti/AttachOnDemand/attach045
        This test failed due to "IOException: well-known file is not
        secure" on Linux IA32 Client VM (machine wowamd). This is an
        occurrence of the following bug:

            6649594 3/4 Intermittent IOExceptions during dynamic attach on linux
        I will copy this entry to 6649594.
We also regularly see intermittent failures in attach-on-demand test with following exception:
java.io.IOException: Bad file number
        at sun.tools.attach.SolarisVirtualMachine.enqueue(Native Method)
        at sun.tools.attach.SolarisVirtualMachine.execute(SolarisVirtualMachine.java:107)
        at sun.tools.attach.HotSpotVirtualMachine.loadAgentLibrary(HotSpotVirtualMachine.java:40)
        at sun.tools.attach.HotSpotVirtualMachine.loadAgentLibrary(HotSpotVirtualMachine.java:61)
        at sun.tools.attach.HotSpotVirtualMachine.loadAgent(HotSpotVirtualMachine.java:85)
        at nsk.share.aod.AgentsAttacher.tryToLoadAgent(AgentsAttacher.java:68)
        at nsk.share.aod.AgentsAttacher.attachAgents(AgentsAttacher.java:48)
        at nsk.share.aod.AODTestRunner.doTestActions(AODTestRunner.java:60)
        at nsk.share.aod.AODTestRunner.runTest(AODTestRunner.java:112)
        at nsk.share.aod.AODTestRunner.main(AODTestRunner.java:144)

I tried to run attach-on-demand tests with -XX:+StartAttachListener and with this flag tests don't fail.

Looks like this bug also affects JDI tests against com.sun.jdi.ProcessAttach connector:
nsk/jdi/AttachingConnector/attach/attach004
nsk/jdi/AttachingConnector/attachnosuspend/attachnosuspend002

Example of tests failures:
http://vmsqe.russia.sun.com/execution/results/JDK7/PROMOTION/VM/b39/ConcMarkSweepGC_2/vm/solaris-i586/server/comp/solaris-i586_server_comp_nsk.jdi.testlist/ResultDir/attachnosuspend002/attachnosuspend002.log
http://vmsqe.russia.sun.com/execution/results/JDK7/PROMOTION/VM/b39/ConcMarkSweepGC_2/vm/solaris-sparcv9/server/mixed/solaris-sparcv9_server_mixed_nsk.jdi.testlist/ResultDir/attach004/attach004.log
Adding a number of entries from my nightly analysis report:

New nsk.jvmti failures (from 2009.01.30)
    nsk/jvmti/AttachOnDemand/attach003
        This test failed due to "ERROR: Unexpected IOException during
        VirtualMachine.attach: java.io.IOException: Permission denied"
        on Solaris AMD64 Server VM (machine vm-v20z-29).

New nsk.jvmti failures (from 2009.01.23)
    nsk/jvmti/AttachOnDemand/attach013
        This test failed due to "IOException: well-known file is not
        secure" on Linux IA32 Server VM -Xmixed (machine jtg-linux17).

New nsk.jvmti failures (from 2009.01.05)
    nsk/jvmti/AttachOnDemand/attach037
        This test failed due to "ERROR: Unexpected IOException during
        VirtualMachine.attach: java.io.IOException: Connection refused"
        on Linux AMD64 Server VM -Xmixed (machine vm-v20z-3).

New nsk.jvmti failures (from 2008.12.12)
    nsk/jvmti/AttachOnDemand/attach020
        This test failed due to "IOException: well-known file is not
        secure" on Linux AMD64 Server VM (machine vmnightly6).

New nsk.jvmti failures (from 2008.11.28)
    nsk/jvmti/AttachOnDemand/attach042
        This test failed due to "ERROR: Unexpected IOException during
        VirtualMachine.attach: java.io.IOException: Permission denied"
        on Solaris SPARC Server VM (machine vm-v215-02).

New nsk.jvmti failures (from 2008.11.26)
    nsk/jvmti/AttachOnDemand/attach010
        This test failed due to "ERROR: Unexpected IOException during
        VirtualMachine.attach: java.io.IOException: Connection refused"
        on Linux IA32 Client VM (machine inclusion).

New nsk.jvmti failures (from 2008.10.30)
    nsk/jvmti/AttachOnDemand/attach014
        This test failed due to "IOException: well-known file is not
        secure" on Linux AMD64 Server VM (machine wowamd).

        Update: In the 2009.01.31 nightly, the test failed due to
            "ERROR: Unexpected IOException during VirtualMachine.attach:
            java.io.IOException: Connection refused" on Linux AMD64
            Server VM -Xcomp (machine intelsdv17).

New nsk.jvmti failures (from 2008.10.01)
    nsk/jvmti/AttachOnDemand/attach012
        This test failed due to "ERROR: Unexpected IOException during
        VirtualMachine.attach: java.io.IOException: Connection refused"
        on Linux IA32 Server VM (machine intelsdv17).