I just ran into the same problem.
Either the documentation or the source should be fixed depending on what the true intended default value is supposed to be.

The slew of warnings are likely because of this bug: http://pecl.php.net/bugs/bug.php?id=18624

Slam defense (if its working properly) should be a good idea to have enabled to help spot issues with your code doing multiple caching of the same object.

Background info:
Ubuntu 10.04.1
PEAR Version: 1.9.1
PHP Version: 5.3.3 (built from source with Suhosin patch 0.9.10)
APC 3.1.3p1 and 3.1.4 (installed via PECL)

Upon upgrading from APC 3.1.3p1 to 3.1.4, I noticed a slew of
"apc_store(): Potential cache slam averted for key" PHP warnings in my
logs.
Despite the APC changelog and runtime configuration doc stating
apc.slam_defense has been deprecated since 3.0.11 and should default to 0,
it is set to "1" by default in 3.1.4. If I understand this option
correctly, such a low number would be the cause of so many PHP warnings.

Configuration and runtime details for the two APC versions:

####

3.1.3p1

APC configuration:

apc.cache_by_default =          1
apc.coredump_unmap =            0
apc.enable_cli =                0
apc.enabled =                   1
apc.file_update_protection =    2
apc.gc_ttl =                    600
apc.include_once_override =     0
apc.max_file_size =             4M
apc.shm_segments=               1
apc.shm_size =                  64
apc.stat =                      1
apc.stat_ctime =                1
apc.ttl =                       3600
apc.write_lock =                1
apc.mmap_file_mask =            /dev/zero
apc.user_ttl =                  3600
apc.user_entries_hint =         300
apc.num_files_hint =            400

Runtime settings as reported by APC INFO:

apc.cache_by_default		1
apc.canonicalize		1
apc.coredump_unmap		0
apc.enable_cli			0
apc.enabled			1
apc.file_md5			0
apc.file_update_protection	2
apc.filters	
apc.gc_ttl			600
apc.include_once_override	0
apc.lazy_classes		0
apc.lazy_functions		0
apc.max_file_size		4M
apc.mmap_file_mask		/dev/zero
apc.num_files_hint		400
apc.preload_path	
apc.report_autofilter		0
apc.rfc1867			0
apc.rfc1867_freq		0
apc.rfc1867_name		APC_UPLOAD_PROGRESS
apc.rfc1867_prefix		upload_
apc.rfc1867_ttl			3600
apc.shm_segments		1
apc.shm_size			64
apc.stat			1
apc.stat_ctime			1
apc.ttl	3600
apc.use_request_time		1
apc.user_entries_hint		300
apc.user_ttl			3600
apc.write_lock			1

####

3.1.4

APC configuration:

apc.cache_by_default =          1
apc.coredump_unmap =            0
apc.enable_cli =                0
apc.enabled =                   1
apc.file_update_protection =    2
apc.gc_ttl =                    600
apc.include_once_override =     0
apc.max_file_size =             4M
apc.shm_segments=               1
apc.shm_size =                  64M
apc.stat =                      1
apc.stat_ctime =                1
apc.ttl =                       3600
apc.write_lock =                1
apc.mmap_file_mask =            /dev/zero
apc.user_ttl =                  3600
apc.user_entries_hint =         300
apc.num_files_hint =            400

Runtime settings as reported by APC INFO:

apc.cache_by_default		1
apc.canonicalize		1
apc.coredump_unmap		0
apc.enable_cli			0
apc.enabled			1
apc.file_md5			0
apc.file_update_protection	2
apc.filters	
apc.gc_ttl			600
apc.include_once_override	0
apc.lazy_classes		0
apc.lazy_functions		0
apc.max_file_size		4M
apc.mmap_file_mask		/dev/zero
apc.num_files_hint		400
apc.preload_path	
apc.report_autofilter		0
apc.rfc1867			0
apc.rfc1867_freq		0
apc.rfc1867_name		APC_UPLOAD_PROGRESS
apc.rfc1867_prefix		upload_
apc.rfc1867_ttl	3600
apc.shm_segments		1
apc.shm_size			64M
apc.slam_defense		1
apc.stat			1
apc.stat_ctime			1
apc.ttl				3600
apc.use_request_time		1
apc.user_entries_hint		300
apc.user_ttl			3600
apc.write_lock			1

####

As you can see, in neither configuration is apc.slam_defense set. APC INFO
does not list the option with 3.1.3p1 and shows it set to "1" with 3.1.4.

The PHP warnings have ceased since adding "apc.slam_defense = 0" to my APC
3.1.4 configuration.

As of 3.1.6, application behavior which previously generated "apc_store(): Potential cache slam averted for key" errors no longer does so. Perhaps due to the changes in SVN revision 305258.

Whatever the reason, I believe this is merely a documentation issue and have opened a bug report at http://bugs.php.net/bug.php?id=53538.