I just saw this in a cluster that has been running fine for a long time. Looking at the code, it is checking if a file exists and then later rechecking the existence, generating this error if the file now exists. But with multiple container-server processes running, this is a race condition that should be expected to happen sometimes.  If this error matters, the code should really use create file inside a try rather than "check and create if not there". If the error is harmless it would help to make the message DEBUG or INFO rather than ERROR. Here is the log snippet for the transaction that had this error:

Jan 17 16:41:47 se06-1eth0 account-server 172.24.3.121 - - [17/Jan/2013:21:41:47 +0000] "HEAD /sdb1/20971/AUTH_94daee7efc9a4bd18170feac446d85d2" 204 - "txa02b236b1ce24efcbc4a901538336164" "-" "-" 0.0019 ""
Jan 17 16:41:47 se05-1 account-server 172.24.3.151 - - [17/Jan/2013:21:41:47 +0000] "PUT /sdb1/20971/AUTH_94daee7efc9a4bd18170feac446d85d2/unauthenticated_webtester_420fde67-54b0-41ea-9e3e-b976755783b6" 201 - "txa02b236b1ce24efcbc4a901538336164" "-" "-" 0.0016 ""
Jan 17 16:41:47 se07-1 account-server 172.24.3.161 - - [17/Jan/2013:21:41:47 +0000] "PUT /sdb1/20971/AUTH_94daee7efc9a4bd18170feac446d85d2/unauthenticated_webtester_420fde67-54b0-41ea-9e3e-b976755783b6" 201 - "txa02b236b1ce24efcbc4a901538336164" "-" "-" 0.0016 ""
Jan 17 16:41:47 se05-1 container-server 172.24.3.121 - - [17/Jan/2013:21:41:47 +0000] "PUT /sdc1/31409/AUTH_94daee7efc9a4bd18170feac446d85d2/unauthenticated_webtester_420fde67-54b0-41ea-9e3e-b976755783b6" 201 - "txa02b236b1ce24efcbc4a901538336164" "-" "-" 0.0100
Jan 17 16:41:47 se06-1eth0 container-server 172.24.3.121 - - [17/Jan/2013:21:41:47 +0000] "PUT /sdc1/31409/AUTH_94daee7efc9a4bd18170feac446d85d2/unauthenticated_webtester_420fde67-54b0-41ea-9e3e-b976755783b6" 201 - "txa02b236b1ce24efcbc4a901538336164" "-" "-" 0.0101
Jan 17 16:41:50 se08-1eth0 container-server ERROR __call__ error with PUT /sdb1/31409/AUTH_94daee7efc9a4bd18170feac446d85d2/unauthenticated_webtester_420fde67-54b0-41ea-9e3e-b976755783b6 : #012Traceback (most recent call last):#012  File "/usr/lib/python2.7/dist-packages/swift/container/server.py", line 499, in __call__#012    res = method(req)#012  File "/usr/lib/python2.7/dist-packages/swift/common/utils.py", line 1350, in wrapped#012    return func(*a, **kw)#012  File "/usr/lib/python2.7/dist-packages/swift/container/server.py", line 232, in PUT#012    broker.initialize(timestamp)#012  File "/usr/lib/python2.7/dist-packages/swift/common/db.py", line 247, in initialize#012    'DB created by someone else while working?')#012DatabaseConnectionError: DB connection error (/srv/node/sdb1/containers/31409/0cf/f5630dac212afb5eb9a2912ffc9ae0cf/f5630dac212afb5eb9a2912ffc9ae0cf.db, 0):#012DB created by someone else while working? (txn: txa02b236b1ce24efcbc4a901538336164)
Jan 17 16:41:50 se08-1eth0 container-server 172.24.3.121 - - [17/Jan/2013:21:41:50 +0000] "PUT /sdb1/31409/AUTH_94daee7efc9a4bd18170feac446d85d2/unauthenticated_webtester_420fde67-54b0-41ea-9e3e-b976755783b6" 500 713 "txa02b236b1ce24efcbc4a901538336164" "-" "-" 2.7294
Jan 17 16:41:50 se02-1eth0 proxy-server Handoff requested (1) (txn: txa02b236b1ce24efcbc4a901538336164) (client_ip: 172.26.0.2)
Jan 17 16:41:50 se06-1eth0 account-server 172.24.3.171 - - [17/Jan/2013:21:41:50 +0000] "PUT /sdb1/20971/AUTH_94daee7efc9a4bd18170feac446d85d2/unauthenticated_webtester_420fde67-54b0-41ea-9e3e-b976755783b6" 201 - "txa02b236b1ce24efcbc4a901538336164" "-" "-" 0.0017 ""
Jan 17 16:41:50 se07-1 container-server 172.24.3.121 - - [17/Jan/2013:21:41:50 +0000] "PUT /sdc1/31409/AUTH_94daee7efc9a4bd18170feac446d85d2/unauthenticated_webtester_420fde67-54b0-41ea-9e3e-b976755783b6" 201 - "txa02b236b1ce24efcbc4a901538336164" "-" "-" 0.0102
Jan 17 16:41:50 se02-1eth0 proxy-server 172.26.0.2 172.24.3.182 17/Jan/2013/21/41/50 PUT /v1/AUTH_94daee7efc9a4bd18170feac446d85d2/unauthenticated_webtester_420fde67-54b0-41ea-9e3e-b976755783b6 HTTP/1.0 201 - - c6e886fc0b324fa6bf2383c084d15587 - 18 - txa02b236b1ce24efcbc4a901538336164 - 2.7502 -