Java on Linux sometimes produces libxcb locking errors, such as:

java: xcb_xlib.c:50: xcb_xlib_unlock: Assertion `c->xlib.lock' failed.

or the following:

Locking assertion failure. Backtrace:
#0 /usr/lib/libxcb-xlib.so.0 [0xb737d767]
#1 /usr/lib/libxcb-xlib.so.0(xcb_xlib_unlock+0x31) [0xb737d8b1]
#2 /usr/lib/libX11.so.6(_XReply+0xfd) [0x8c76e1bd]

This is due to a bug in Sun's JVM that we cannot fix:
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6532373

More discussion and workarounds can be found in the following discussion
threads:
http://bugs.gentoo.org/show_bug.cgi?id=156353
http://forums.opensuse.org/archives/sf-archives/archives-programming-scripting/342666-java-6-libxcb-solved.html
http://processing.org/discourse/yabb_beta/YaBB.cgi?board=Integrate;action=display;num=1210090458

Starting with release 0150, we'll try to include a later version of Java
(1.6.0_06 or later) that fixes the problem as part of the download.

...and the solution in the meantime is to follow the instructions in the
Linux reference to install and symlink a better JDK for your platform:
http://processing.org/reference/environment/platforms.html#linux

*** Bug 897 has been marked as a duplicate of this bug. ***

Release 0150 now uses Java 1.6.0_07 as its default runtime, which seems to
fix the problem at least on Ubuntu 8.04.