<p dir="auto">When concatting two dataframes where there are a) there are duplicate columns in one of the dataframes, and b) there are non-overlapping column names in both, then you get a IndexError:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="In [9]: df1 = pd.DataFrame(np.random.randn(3,3), columns=['A', 'A', 'B1'])
   ...: df2 = pd.DataFrame(np.random.randn(3,3), columns=['A', 'A', 'B2'])

In [10]: pd.concat([df1, df2])

Traceback (most recent call last):
  File &quot;&lt;ipython-input-10-f61a1ab4009e&gt;&quot;, line 1, in &lt;module&gt;
    pd.concat([df1, df2])
...
  File &quot;c:\users\vdbosscj\scipy\pandas-joris\pandas\core\index.py&quot;, line 765, in take
    taken = self.view(np.ndarray).take(indexer)
IndexError: index 3 is out of bounds for axis 0 with size 3"><pre class="notranslate"><code class="notranslate">In [9]: df1 = pd.DataFrame(np.random.randn(3,3), columns=['A', 'A', 'B1'])
   ...: df2 = pd.DataFrame(np.random.randn(3,3), columns=['A', 'A', 'B2'])

In [10]: pd.concat([df1, df2])

Traceback (most recent call last):
  File "&lt;ipython-input-10-f61a1ab4009e&gt;", line 1, in &lt;module&gt;
    pd.concat([df1, df2])
...
  File "c:\users\vdbosscj\scipy\pandas-joris\pandas\core\index.py", line 765, in take
    taken = self.view(np.ndarray).take(indexer)
IndexError: index 3 is out of bounds for axis 0 with size 3
</code></pre></div>
<p dir="auto">I don't know if it should work (although I suppose it should, as with only the duplicate columns it does work), but at least the error message is not really helpfull.</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="dti = pd.date_range('2016-09-23', periods=3, tz='US/Central')
df = pd.DataFrame(dti)

&gt;&gt;&gt; df.T
Traceback (most recent call last):
  File &quot;&lt;stdin&gt;&quot;, line 1, in &lt;module&gt;
  File &quot;pandas/core/frame.py&quot;, line 1941, in transpose
    return super(DataFrame, self).transpose(1, 0, **kwargs)
  File &quot;pandas/core/generic.py&quot;, line 616, in transpose
    new_values = self.values.transpose(axes_numbers)
  File &quot;pandas/core/base.py&quot;, line 697, in transpose
    nv.validate_transpose(args, kwargs)
  File &quot;pandas/compat/numpy/function.py&quot;, line 54, in __call__
    self.defaults)
  File &quot;pandas/util/_validators.py&quot;, line 218, in validate_args_and_kwargs
    validate_kwargs(fname, kwargs, compat_args)
  File &quot;pandas/util/_validators.py&quot;, line 157, in validate_kwargs
    _check_for_default_values(fname, kwds, compat_args)
  File &quot;pandas/util/_validators.py&quot;, line 69, in _check_for_default_values
    format(fname=fname, arg=key)))
ValueError: the 'axes' parameter is not supported in the pandas implementation of transpose()"><pre class="notranslate"><code class="notranslate">dti = pd.date_range('2016-09-23', periods=3, tz='US/Central')
df = pd.DataFrame(dti)

&gt;&gt;&gt; df.T
Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  File "pandas/core/frame.py", line 1941, in transpose
    return super(DataFrame, self).transpose(1, 0, **kwargs)
  File "pandas/core/generic.py", line 616, in transpose
    new_values = self.values.transpose(axes_numbers)
  File "pandas/core/base.py", line 697, in transpose
    nv.validate_transpose(args, kwargs)
  File "pandas/compat/numpy/function.py", line 54, in __call__
    self.defaults)
  File "pandas/util/_validators.py", line 218, in validate_args_and_kwargs
    validate_kwargs(fname, kwargs, compat_args)
  File "pandas/util/_validators.py", line 157, in validate_kwargs
    _check_for_default_values(fname, kwds, compat_args)
  File "pandas/util/_validators.py", line 69, in _check_for_default_values
    format(fname=fname, arg=key)))
ValueError: the 'axes' parameter is not supported in the pandas implementation of transpose()
</code></pre></div>
<p dir="auto">Looks like this is b/c <code class="notranslate">df.values</code> is a <code class="notranslate">DatetimeIndex</code></p>