Description:
------------
That's odd. It happens with latest MASTER branch and SVN short after  1.0.1. I have reverted to 1.0.1 to work around it. Freshly tested on SVN + libmemcached 0.40

Scenario: 2 servers (1.4.4), ketama ON, distribution DEF/CONSISTENT (irrelevant), binary ON/OFF (irrelevant), persistent ON.

Operations performed (1st run):
1. fetch KEY -> result: 16 - NOT FOUND
2. add KEY -> result: 0 - SUCCESS
3. [END]

Operations performed (2nd run)
1. fetch KEY -> result: 0 - SUCCESS (found, cas=X)
2. CAS(X,KEY,val) -> result: 16 - NOT FOUND

 ??? confusion, why is it not found ???

3. loop again
4. fetch KEY -> result: 0 - SUCCESS (found, cas=X)

 ??? same CAS, so the value has not changed! ???

5. CAS(X,KEY,val) -> result: 16 - NOT FOUND

 ??? why is it missing ???

6. ..... infinite loop .......


WORKAROUND1: Roll back to 1.0.1 - works flawlessly
WORKAROUND2: Disable KETAMA (?) Seems to work without it.


Below is a code taken from manual, with some added verbosity and a loop iteration limit ($x<2 for debugging).


Reproduce code:
---------------
<?php
$m = new Memcached('slamtest1');
if(!count($m->getServerList())){
	$m->addServers(array(
		array('127.0.0.101',11211),
		array('127.0.0.102',11211),
	));
	$m->setOption(Memcached::OPT_LIBKETAMA_COMPATIBLE, true);
	$m->setOption(Memcached::OPT_DISTRIBUTION,Memcached::DISTRIBUTION_CONSISTENT);
}

echo "Opt: Ketama: ";
var_dump($m->getOption(Memcached::OPT_LIBKETAMA_COMPATIBLE));
echo "Opt: distribution: ";
var_dump($m->getOption(Memcached::OPT_DISTRIBUTION));
echo "Opt: binary protocol: ";
var_dump($m->getOption(Memcached::OPT_BINARY_PROTOCOL));

print_r($m->getServerList());
print_r($m->getVersion());

set_time_limit(5);error_reporting(E_ALL);ini_set('display_errors',1);
ini_set('output_buffering',0);
ob_end_flush();

// -- do a increase test
$key = 'slamtest_1_incr';
$x = 0;
do{
	$x++;
	// -- fetch the value
	$cas = 0.0;
	$success = false;
	echo "Fetching $key\n";flush();
	$val = $m->get($key,null,$cas);
	echo "after fetch, RESULT: ".$m->getResultCode()." - ".$m->getResultMessage()."\n";flush();
	echo "VAL: $val\n";
	if ($m->getResultCode() == Memcached::RES_NOTFOUND) {
		echo "Not found - adding\n";
		// -- value does not exist yet
		if($m->add($key, 1)){
			echo "add success! RESULT: ".$m->getResultCode()." - ".$m->getResultMessage()."\n";flush();
			$success = 1;
		}else{
			echo "add failed! RESULT: ".$m->getResultCode()." - ".$m->getResultMessage()."\n";flush();
		}
	}else{
		echo "Found!\n";flush();
		echo "CAS: (".gettype($cas).") $cas\n";flush();
		// -- increase the value by 1
		$val++;

		// -- store it
		if($m->cas($cas,$key,$val)){
			echo "CAS success\n";flush();
			$success = true;
		}else{
			echo "CAS failed, because ".$m->getResultCode()." - ".$m->getResultMessage()."\n";flush();
		}
	}
	echo "LOOP RESULT: ".$m->getResultCode()." - ".$m->getResultMessage()."\n";;flush();
	if(!$success) usleep(mt_rand(10000,100000));
}while($x<2 && !$success);

echo "Finished after $x iters!\n";exit();


Expected result:
----------------
#### THE VERY SAME CODE RUN ON VER 1.0.1 AND LIBMEMCACHED 0.38 WITH THE SAME PAIR OF MEMCACHED DAEMONS ####
Opt: Ketama: int(1)
Opt: distribution: int(1)
Opt: binary protocol: int(0)
Array
(
    [0] => Array
        (
            [host] => 127.0.0.101
            [port] => 11211
            [weight] => 1
        )

    [1] => Array
        (
            [host] => 127.0.0.102
            [port] => 11211
            [weight] => 1
        )

)
Array
(
    [127.0.0.101:11211] => 1.4.4
    [127.0.0.102:11211] => 1.4.4
)
Fetching slamtest_1_incr
after fetch, RESULT: 0 - SUCCESS
VAL: 3
Found!
CAS: (double) 730849
CAS success
LOOP RESULT: 0 - SUCCESS
Finished (1)!


Actual result:
--------------
#### CODE RUN ON TRUNK + LIBMEMCACHED 0.40 WITH THE SAME PAIR OF MEMCACHED DAEMONS ####
Opt: Ketama: int(1)
Opt: distribution: int(1)
Opt: binary protocol: int(0)
Array
(
    [0] => Array
        (
            [host] => 127.0.0.101
            [port] => 11211
            [weight] => 1
        )

    [1] => Array
        (
            [host] => 127.0.0.102
            [port] => 11211
            [weight] => 1
        )

)
Array
(
    [127.0.0.101:11211] => 1.4.4
    [127.0.0.102:11211] => 1.4.4
)
Fetching slamtest_1_incr
after fetch, RESULT: 0 - SUCCESS
VAL: 3
Found!
CAS: (double) 641304
CAS failed, because 16 - NOT FOUND
LOOP RESULT: 16 - NOT FOUND
Fetching slamtest_1_incr
after fetch, RESULT: 0 - SUCCESS
VAL: 3
Found!
CAS: (double) 641304
CAS failed, because 16 - NOT FOUND
LOOP RESULT: 16 - NOT FOUND
Finished (2)!