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I managed to reproduce this again, this time by corrupting data on a btrfs filesystem.
$ cat read_error_file.py 
import os
fname = "/usr/bin/Xorg"
size = os.stat(fname).st_size
print fname, "stat size:", size
f = open(fname, "rb")
print "len(f.read()): ", len(f.read())
f.close()
f = open(fname, "rb")
for i in xrange(size):
 try:
 f.read(1)
 except IOError:
 print "IOError at byte %d" % i
 break
f.close()
$ python read_error_file.py 
/usr/bin/Xorg stat size: 2331776
len(f.read()): 716800
IOError at byte 716800
Note how the first test does not throw an IOError, but the second one does.
The strace for the first test is:
open("/usr/bin/Xorg", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0755, st_size=2331776, ...}) = 0
fstat(3, {st_mode=S_IFREG|0755, st_size=2331776, ...}) = 0
lseek(3, 0, SEEK_CUR) = 0
fstat(3, {st_mode=S_IFREG|0755, st_size=2331776, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f1334bd6000
lseek(3, 0, SEEK_CUR) = 0
mmap(NULL, 2334720, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f1332ea6000
read(3, "177円ELF2円1円1円0円0円0円0円0円0円0円0円0円3円0円>0円1円0円0円0円265円M4円0円0円0円0円0円"..., 2330624) = 716800
read(3, 0x7f1332f55034, 1613824) = -1 EIO (Input/output error)
mremap(0x7f1332ea6000, 2334720, 720896, MREMAP_MAYMOVE) = 0x7f1332ea6000
munmap(0x7f1332ea6000, 720896) = 0
write(1, "len(f.read()): 716800\n", 23len(f.read()): 716800
) = 23
Note the "-1 EIO (Input/output error)" that gets ignored somewhere.