The NUnit plugin occasionally under-reports the total number of tests and failures.
I have two seperate installs of Jenkins, each with NUnit 1.3, both of which are occasionally showing the number of tests (and failures, if there are any) as much smaller than they should be.

The example shows my most recent build info (TestResults-current.xml and junitResult-current.xml) and the previous build's results (junitResults-previous.xml). The previous build correctly reported the number of tests and failures. The current build is showing about 4000 fewer tests than it should.