GitHub #101: Document invalid combination of parameters.
diff --git a/org.jacoco.doc/docroot/doc/ant.html b/org.jacoco.doc/docroot/doc/ant.html
index 74dedfa..5098c1e 100644
--- a/org.jacoco.doc/docroot/doc/ant.html
+++ b/org.jacoco.doc/docroot/doc/ant.html
@@ -137,7 +137,10 @@
In addition the <code>junit</code> task should declare
<code>forkmode="once"</code> to avoid starting a new JVM for every single test
case and decreasing execution performance dramatically (unless this is
- required by the nature of the test cases).
+ required by the nature of the test cases). Note that
+ <code>forkmode="perTest"</code> or <code>forkmode="perBatch"</code> should not
+ be combined with <code>append="false"</code> as the execution data file is
+ overwritten with the execution of every test.
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