I have apt-get-installed libnss-3.12.9.0-alt1.M50P.1 from p5 branch in an older system (I had libnss-3.12.5.0-alt1.20090918 there before).

In the result, the dependencies of the new libnss were not satisfied:

$ chromium-browser 
[6676:6687:5191509052:ERROR:nss_util.cc(346)] NSS_VersionCheck("3.12.3") failed.  We depend on NSS >= 3.12.3, and this error is not fatal only because many people have busted NSS setups (for example, using the wrong version of NSPR). Please upgrade to the latest NSS and NSPR, and if you still get this error, contact your distribution maintainer.
/usr/lib/chromium-browser/chromium-browser: symbol lookup error: /usr/lib/libnssutil3.so: undefined symbol: PL_ClearArenaPool
$ rpm -qf /usr/lib/libnssutil3.so 
libnss-3.12.9.0-alt1.M50P.1
$ ldd -r /usr/lib/libnssutil3.so 
undefined symbol: PL_ClearArenaPool	(/usr/lib/libnssutil3.so)
	linux-gate.so.1 =>  (0xb7fc2000)
	libplc4.so => /usr/lib/libplc4.so (0xb7f7a000)
	libplds4.so => /usr/lib/libplds4.so (0xb7f75000)
	libnspr4.so => /usr/lib/libnspr4.so (0xb7f31000)
	libc.so.6 => /lib/libc.so.6 (0xb7dd9000)
	libpthread.so.0 => /lib/libpthread.so.0 (0xb7dbf000)
	libdl.so.2 => /lib/libdl.so.2 (0xb7dba000)
	/lib/ld-linux.so.2 (0xb7fc3000)
$ 

Perhaps, adding more strict formal dependencies to the libnss package would be beneficial for those who are upgrading their systems to p5.

More info (about my old libs there):

$ ldd -r /usr/lib/libnssutil3.so | awk '/=> \// { print $3 }' | xargs rpm -qf | sort -u
undefined symbol: PL_ClearArenaPool	(/usr/lib/libnssutil3.so)
glibc-core-2.11.2-alt1.M51.1
glibc-pthread-2.11.2-alt1.M51.1
libnspr-4.8.0-alt1
$