I added the AmpersandHQ debug code this morning and just now had the "Front controller reached 100 router match iterations" exception happen about 75 times in a 2 minute period. But this time, presumably because of the debug code not saving the corrupt cache entry, the site is still up without everyone getting exceptions (I did not flush the cache).
I have not dug into this yet to investigate but corrupt-cache.log contains:
2015-11-22T03:42:42+00:00 DEBUG (7):
#0 app/code/core/Mage/Core/Model/App.php(1147): Mage_Core_Model_Cache->save('<admin><design>...', 'config_global_s...', Array, NULL)
#1 app/code/core/Mage/Core/Model/Config.php(552): Mage_Core_Model_App->saveCache('<admin><design>...', 'config_global_s...', Array, NULL)
#2 app/code/core/Mage/Core/Model/Config.php(474): Mage_Core_Model_Config->_saveCache('<admin><design>...', 'config_global_s...', Array, NULL)
#3 app/code/core/Mage/Core/Model/App.php(421): Mage_Core_Model_Config->saveCache()
#4 app/code/core/Mage/Core/Model/App.php(343): Mage_Core_Model_App->_initModules()
#5 app/Mage.php(683): Mage_Core_Model_App->run(Array)
#6 index.php(87): Mage::run('', 'store')
#7 {main}
This is on Magento 1.7.0.2 with Redis cache and AmpersandHQ's SUPEE-4755 patch already applied.
Dec 2, 2015 update: Here is another error with the full stack trace:
2015-12-02T20:02:27+00:00 DEBUG (7):
#0 app/code/local/Mage/Core/Model/App.php(1156): save('<admin><design><package><name>default</name></package><theme><default>find</default></theme></design></admin>', 'config_global_stores_admin', Array, NULL)
#1 app/code/local/Mage/Core/Model/Config.php(552): saveCache('<admin><design><package><name>default</name></package><theme><default>find</default></theme></design></admin>', 'config_global_stores_admin', Array, NULL)
#2 app/code/local/Mage/Core/Model/Config.php(474): _saveCache('<admin><design><package><name>default</name></package><theme><default>find</default></theme></design></admin>', 'config_global_stores_admin', Array, NULL)
#3 app/code/local/Mage/Core/Model/App.php(430): saveCache()
#4 app/code/local/Mage/Core/Model/App.php(343): _initModules()
#5 app/Mage.php(683): run(Array)
#6 index.php(87): run('', 'store')