FULL PRODUCT VERSION :
java version "1.6.0_16"
Java(TM) SE Runtime Environment (build 1.6.0_16-b01)
Java HotSpot(TM) Client VM (build 14.2-b01, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :
java.lang.String#contentEquals(StringBuffer) synchronizes on the given StringBuffer. This is correct.

java.lang.String#contentEquals(CharSequence) does not synchronize on StringBuffers. This is not correct.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Look at the source code.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
str.contentEquals((CharSequence) new StringBuffer()) should synchronize on the given StringBuffer.
ACTUAL -
str.contentEquals((CharSequence) new StringBuffer()) does not synchronize on the given StringBuffer.

REPRODUCIBILITY :
This bug can be reproduced always.