I did test on 
- Python 3.5.1 on Windows 64 Bit and 32 Bit (Machine A)
- Python 3.4.4 on Windows 32 Bit (Machine A)
- Python 3.4.3+ on Ubuntu 15.10 64 Bit (Virtual Machine on Machine A)
- Python 3.4.2 on Machine B and C
- Python 3.3.5 on Windows 32 Bit (Machine A)

I did test but not produce bug. (report 8 correctly)
- Python 3.2.5 on Windows 32 Bit (Machine A)
- Python 3.1.4 on Windows 32 Bit (Machine A)
- Python 2.7.10 on Windows 64 Bit (Machine A) 

Machine A: i7-5775C with Windows 10 (build 10586.36) 64 Bit
Machine B: http://www.tutorialspoint.com/execute_python3_online.php
Machine C: https://repl.it/languages/python3

Code are here

import io
with io.TextIOWrapper(io.BytesIO(b'.\r\n...\r\n\r\n\r\n')) as fp:
    fp.readline() # '.\n'
    fp.readline() # '......\n'
    print(fp.tell()) # 18446744073709551628 = 0x10000000000000009

Not only those string produce bug, also adding more dot make produce bug sometimes.

Yeah, it's expected that tell() can return big numbers. It returns a black box "cookie".

https://docs.python.org/dev/library/io.html#id3

https://docs.python.org/dev/library/io.html#io.TextIOWrapper

second fp.readline() # '......\n' is actaully '...\n'
mistake

The documentation for tell() <https://docs.python.org/3/library/io.html#io.TextIOBase.tell> says that it returns an opaque number. Depending on the codec used, it might have to record the state of the text decoder, CRLF decoder, byte position, etc, all in that number. So I don’t think the return value being large is a bug.

Perhaps the change in behaviour from Python 2 is due to Issue 11114. Also, Issue 25849 was recently opened about text file seeking.

EcmaXp: Do you think there is anything we can do for this? I think the cases discussed here are working as intended.