OK, absolutely stumped. I thought maybe it's a race condition, as the Delete button is greyed out until the username is checked (and it does get checked), but even a sleep(10000) and a log call don't seem to fix that. Any selenium jockeys sensing anything familiar here?

...
    [junit] 15:51:31,250 Checking for link with exact text 'Projects'
    [junit] 15:51:31,265 Checking for link with exact text 'Site Users'
    [junit] 15:51:31,265 Clicking link with text 'Site Users'
    [junit] 15:51:31,281 selenium - Clicking on element: link=Site Users
    [junit] 15:51:31,796 Checking checkbox //a[text()="details"]/../../td[text()="delete_me@security.test"]/../td[1]/input
    [junit] 15:51:31,796 selenium - Clicking on element: //a[text()="details"]/.
./../td[text()="delete_me@security.test"]/../td[1]/input
    [junit] 15:51:31,828 trying to delete user [bpratt added log message]...
    [junit] 15:51:43,187 =============== Completed SecurityTest (1 of 1) =================
    [junit]
    [junit] testSteps(org.labkey.test.drt.SecurityTest)
    [junit] junit.framework.AssertionFailedError: No button found with text "Delete"
    [junit] at org.labkey.test.BaseSeleniumWebTest.clickButton(BaseSeleniumWebTest.java:2069)
    [junit] at org.labkey.test.BaseSeleniumWebTest.clickNavButton(BaseSeleniumWebTest.java:2093)
...