Exception "No successful match so far" after Matcher.find() == true

Matcher m = patternFoo.matcher(sFoo);

if(m.find()){
...
m.start()
...
}



I wonder how it is possible for the m.start() in the above code to throw the following exception:
Exception: No successful match so far
Class: java.lang.IllegalStateException
Stack trace: java.lang.IllegalStateException: No successful match so far
at java.util.regex.Matcher.ensureMatch(Matcher.java:1116)
at java.util.regex.Matcher.start(Matcher.java:1158)
at java.util.regex.Matcher.start(Matcher.java:1130)



Could anyone shed some light on this?