Python 3.5.1+ (3.5:014e6f7d7c1a, Dec 14 2015, 11:20:58) 
[GCC 5.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from io import *
>>> text = TextIOWrapper(BytesIO(b"x"), "ascii")
>>> text.read(1)
'x'
>>> text.read()
''
>>> text.seek(0, SEEK_CUR)
python: ./Modules/_io/textio.c:2293: _io_TextIOWrapper_tell_impl: Assertion `self->decoded_chars == ((void *)0) || PyUnicode_GetLength(self->decoded_chars) == 0' failed.
Aborted (core dumped)
[Exit 134]

May affect other versions; I haven’t looked.

Similar case:

text.read(1); text.write(b'x'); text.tell()

And there may be crashes after setting self->snapshot to NULL due to the failure of Py_BuildValue().

New changeset 23db935bcf258657682e66464bf8512def8af830 by Serhiy Storchaka (Zackery Spytz) in branch 'master':
bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918)
https://github.com/python/cpython/commit/23db935bcf258657682e66464bf8512def8af830

New changeset eabebbb54c2604fd8d08e8019ea1be634aed8c2f by Miss Islington (bot) in branch '3.7':
bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918)
https://github.com/python/cpython/commit/eabebbb54c2604fd8d08e8019ea1be634aed8c2f

New changeset d6a283b37b66ecd2d0ff43602ddc8e91b54a51c5 by Serhiy Storchaka (Zackery Spytz) in branch '3.6':
[3.6] bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918). (GH-8012)
https://github.com/python/cpython/commit/d6a283b37b66ecd2d0ff43602ddc8e91b54a51c5

New changeset 0464de0f9a226cfa32b803e0326c12b2432aba26 by Serhiy Storchaka (Zackery Spytz) in branch '2.7':
[2.7] bpo-25862: Fix assertion failures in io.TextIOWrapper.tell(). (GH-3918). (GH-8013)
https://github.com/python/cpython/commit/0464de0f9a226cfa32b803e0326c12b2432aba26

The assertion failure can be also caused by setting self->snapshot to NULL when Py_BuildValue() fails due to lack of memory. It is very difficult to reproduce this. PR 8026 fixes this and other difficulty reproducible refcount bugs in the _io module.

New changeset fdb5a50ef34f7951c3b01eb77b1359725a9ad670 by Serhiy Storchaka in branch 'master':
bpo-25862: Fix several bugs in the _io module. (GH-8026)
https://github.com/python/cpython/commit/fdb5a50ef34f7951c3b01eb77b1359725a9ad670

New changeset dedb28efd84314d899f795e821a3fc27d2f928e7 by Miss Islington (bot) in branch '3.7':
bpo-25862: Fix several bugs in the _io module. (GH-8026)
https://github.com/python/cpython/commit/dedb28efd84314d899f795e821a3fc27d2f928e7

New changeset e5153d0d404d58e83d802199bea1a8dd0ea5178e by Miss Islington (bot) in branch '3.6':
bpo-25862: Fix several bugs in the _io module. (GH-8026)
https://github.com/python/cpython/commit/e5153d0d404d58e83d802199bea1a8dd0ea5178e

New changeset eab421bff954e4fb77516bfe6c98d30ced1412d0 by Serhiy Storchaka in branch '2.7':
[2.7] bpo-25862: Fix several bugs in the _io module. (GH-8026) (GH-8033)
https://github.com/python/cpython/commit/eab421bff954e4fb77516bfe6c98d30ced1412d0

I haven’t confirmed, but it looks like the original change here broke bidirectional reading and writing with a single TextIOWrapper object; see Issue 35928.