buildfarm: could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes

We've seen several occurrences of $subject in the buildfarm in the past
month or so.  Scraping the logs, I find

 coypu    | 2018-06-14 21:17:49 | HEAD          | Check | 2018-06-14 23:31:44.505 CEST [5b22deb8.30e1:124] ERROR:
couldnot read block 3 in file "base/16384/2662": read only 0 of 8192 bytes 
 coypu    | 2018-06-14 21:17:49 | HEAD          | Check | 2018-06-14 23:31:44.653 CEST [5b22deb8.6d4d:132] ERROR:
couldnot read block 3 in file "base/16384/2662": read only 0 of 8192 bytes 
 gull     | 2018-06-21 04:02:11 | HEAD          | Check | 2018-06-20 21:27:06.843 PDT [31138:3] FATAL:  could not read
block3 in file "base/16384/2662": read only 0 of 8192 bytes 
 mandrill | 2018-06-22 16:14:16 | HEAD          | Check | 2018-06-22 16:46:24.138 UTC [14353240:43]
pg_regress/create_table_likeERROR:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes 
 mandrill | 2018-06-22 16:14:16 | HEAD          | Check | 2018-06-22 16:46:24.137 UTC [20710034:1] ERROR:  could not
readblock 3 in file "base/16384/2662": read only 0 of 8192 bytes 
 lapwing  | 2018-07-05 07:20:02 | HEAD          | Check | 2018-07-05 07:21:45.585 UTC [24022:1] ERROR:  could not read
block3 in file "base/16384/2662": read only 0 of 8192 bytes 
 lapwing  | 2018-07-05 07:20:02 | HEAD          | Check | 2018-07-05 07:21:45.591 UTC [23941:39]
pg_regress/roleattributesERROR:  could not read block 3 in file "base/16384/2662": read only 0 of 8192 bytes 
 mantid   | 2018-07-17 01:20:04 | REL_11_STABLE | Check | 2018-07-16 21:21:32.557 EDT [26072:1] ERROR:  could not read
block3 in file "base/16384/2662": read only 0 of 8192 bytes 
 mantid   | 2018-07-17 01:20:04 | REL_11_STABLE | Check | 2018-07-16 21:21:32.557 EDT [25990:82] ERROR:  could not read
block3 in file "base/16384/2662": read only 0 of 8192 bytes 
 coypu    | 2018-07-17 01:27:39 | HEAD          | Check | 2018-07-17 03:47:26.264 CEST [5b4d4aa4.3ac4:71] ERROR:  could
notread block 3 in file "base/16384/2662": read only 0 of 8192 bytes 
 coypu    | 2018-07-20 11:18:02 | HEAD          | Check | 2018-07-20 13:32:14.104 CEST [5b51c833.4884:131] ERROR:
couldnot read block 3 in file "base/16384/2662": read only 0 of 8192 bytes 

While I didn't go back indefinitely far, there are no other occurrences of
"could not read block" failures in the last three months.  This suggests
strongly that we broke something in early June.  Don't know what though.
Ideas?

In case anyone's wondering, 2662 is pg_class_oid_index.

            regards, tom lane