So apparently, this is what happens when you update Composer at the same moment as you're running a composer update...

gistfile1.txt

update-deps:
     [echo] Composer update
     [exec] Warning: This development build of composer is over 30 days old. It is recommended to update it by running "/usr/local/bin/composer self-update" to get the latest version.
     [exec] Loading composer repositories with package information
     [exec] Updating dependencies (including require-dev)
     [exec] e)
     [exec] {
     [exec] $this->addReason($rule->getId(), array(
     [exec] 'rule' => $rule,
     [exec] 'job' => $rule->getJob(),
     [exec] ));
     [exec] }
     [exec] 
     [exec] 
     [exec] 
     [exec] 
     [exec] 
     [exec] 
     [exec] public function getReasons()
     [exec] {
     [exec] return $this->reasons;
     [exec] }
     [exec] 
     [exec] 
     [exec] 
     [exec] 
     [exec] 
     [exec] 
     [exec] 
     [exec] public function getPrettyString(array $installedMap = array())
     [exec] {
     [exec] $reasons = call_user_func_array('array_merge', array_reverse($this->reasons));
     [exec] 
     [exec] if (count($reasons) === 1) {
     [exec] reset($reasons);
     [exec] $reason = current($reasons);
     [exec] 
     [exec] $rule = $reason['rule'];
     [exec] $job = $reason['job'];
     [exec] 
     [exec] if (isset($job['constraint'])) {
     [exec] $packages = $this->pool->whatProvides($job['packageName'], $job['constraint']);
     [exec] } else {
     [exec] $packages = array();
     [exec] }
     [exec] 
     [exec] if ($job && $job['cmd'] === 'install' && empty($packages)) {
     [exec] 
     [exec]  if (0 === stripos($job['packageName'], 'ext-')) {
     [exec] $ext = substr($job['packageName'], 4);
     [exec] $error = extension_loaded($ext) ? 'has the wrong version ('.(phpversion($ext) ?: '0').') installed' : 'is missing from your system';
     [exec] 
     [exec] return "\n    - The requested PHP extension ".$job['packageName'].$this->constraintToText($job['constraint']).' '.$error.'.';
     [exec] }
     [exec] 
     [exec] 
     [exec]  if (0 === stripos($job['packageName'], 'lib-')) {
     [exec] if (strtolower($job['packageName']) === 'lib-icu') {
     [exec] $error = extension_loaded('intl') ? 'has the wrong version installed, try upgrading the intl extension.' : 'is missing from your system, make sure the intl extension is loaded.';
     [exec] 
     [exec] return "\n    - The requested linked library ".$job['packageName'].$this->constraintToText($job['constraint']).' '.$error;
     [exec] }
     [exec] 
     [exec] return "\n    - The requested linked library ".$job['packageName'].$this->constraintToText($job['constraint']).' has the wrong version installed or is missing from your system, make sure to load the extension providing it.';
     [exec] }
     [exec] 
     [exec] if (!preg_match('{^[A-Za-z0-9_./-]+$}', $job['packageName'])) {
     [exec] $illegalChars = preg_replace('{[A-Za-z0-9_./-]+}', '', $job['pac
     [exec] Fatal error: Class 'Composer\DependencyResolver\RuleWatchGraph' not found in phar:///usr/local/bin/composer/src/Composer/DependencyResolver/Solver.php on line 168
     [exec] 
     [exec] Call Stack:
     [exec]     0.0002     223696   1. {main}() /usr/local/bin/composer:0
     [exec]     0.0093     395144   2. require('phar:///usr/local/bin/composer/bin/composer') /usr/local/bin/composer:15
     [exec]     0.0372    2906656   3. Composer\Console\Application->run() phar:///usr/local/bin/composer/bin/composer:43
     [exec]     0.0401    3204008   4. Symfony\Component\Console\Application->run() phar:///usr/local/bin/composer/src/Composer/Console/Application.php:83
     [exec]     0.0412    3328160   5. Composer\Console\Application->doRun() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:121
     [exec]     0.0422    3411792   6. Symfony\Component\Console\Application->doRun() phar:///usr/local/bin/composer/src/Composer/Console/Application.php:117
     [exec]     0.0423    3412432   7. Symfony\Component\Console\Application->doRunCommand() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:191
     [exec]     0.0423    3412944   8. Symfony\Component\Console\Command\Command->run() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:892
     [exec]     0.0428    3416544   9. Composer\Command\UpdateCommand->execute() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Command/Command.php:241
     [exec]     0.1741    7663560  10. Composer\Installer->run() phar:///usr/local/bin/composer/src/Composer/Command/UpdateCommand.php:12PHP Fatal error:  Class 'Composer\DependencyResolver\RuleWatchGraph' not found in phar:///usr/local/bin/composer/src/Composer/De3
     [exec]     0.1767    7757744  11. Composer\Installer->doInstall() phar:///usr/local/bin/composer/src/Composer/Installer.php:215
     [exec]     2.4978   27702032  12. Composer\DependencyResolver\Solver->solve() phar:///usr/local/bin/composer/src/Composer/Installer.php:467
     [exec] 
     [exec] pendencyResolver/Solver.php on line 168
     [exec] PHP Stack trace:
     [exec] PHP   1. {main}() /usr/local/bin/composer:0
     [exec] PHP   2. require() /usr/local/bin/composer:15
     [exec] PHP   3. Composer\Console\Application->run() phar:///usr/local/bin/composer/bin/composer:43
     [exec] PHP   4. Symfony\Component\Console\Application->run() phar:///usr/local/bin/composer/src/Composer/Console/Application.php:83
     [exec] PHP   5. Composer\Console\Application->doRun() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:121
     [exec] PHP   6. Symfony\Component\Console\Application->doRun() phar:///usr/local/bin/composer/src/Composer/Console/Application.php:117
     [exec] PHP   7. Symfony\Component\Console\Application->doRunCommand() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:191
     [exec] PHP   8. Symfony\Component\Console\Command\Command->run() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Application.php:892
     [exec] PHP   9. Composer\Command\UpdateCommand->execute() phar:///usr/local/bin/composer/vendor/symfony/console/Symfony/Component/Console/Command/Command.php:241
     [exec] PHP  10. Composer\Installer->run() phar:///usr/local/bin/composer/src/Composer/Command/UpdateCommand.php:123
     [exec] PHP  11. Composer\Installer->doInstall() phar:///usr/local/bin/composer/src/Composer/Installer.php:215
     [exec] PHP  12. Composer\DependencyResolver\Solver->solve() phar:///usr/local/bin/composer/src/Composer/Installer.php:467