title: How to back fillna only one na value with a specific value
body: <p>I want to back fillna, but I only want to backfill only one na value and replace that value with a specific value (1)</p>
  
  <p>I tried using </p>
  
  <pre><code>df.fillna(value=1,method='bfill',inplace=True,limit=1)
  </code></pre>
  
  <p>but I get</p>
  
  <pre><code>ValueError: Cannot specify both 'value' and 'method'.
  </code></pre>
  
  <p>because I cannot use method and value at the same time. If this was possible, I would not be asking this question (pandas possibly should look into this with a new update)</p>
  
  <p>Here is an example:</p>
  
  <pre><code>import pandas as pd
  import numpy as np
  
  col1 = [3,2,2,np.nan,np.nan,np.nan,2,6,np.nan,np.nan,np.nan,6]
  col2 = [8,2,np.nan,np.nan,6,0,np.nan,5,np.nan,6,6,3]
  col3 = [np.nan,np.nan,np.nan,np.nan,6,7,np.nan,1,np.nan,np.nan,3,4]
  df = pd.DataFrame(data=[col1,col2,col3],columns=['col1','col2','col3'])
  
  print(df)
  
  index     col1     col2     col3
  0          3        8       np.nan
  1          2        2       np.nan
  2          2        np.nan  np.nan
  3          np.nan   np.nan  np.nan
  4          np.nan   6       6
  5          np.nan   0       7
  6          2        np.nan  np.nan
  7          6        5       1
  8          np.nan   np.nan  np.nan
  9          np.nan   6       np.nan
  10         np.nan   6       3
  11         6        3       4
  </code></pre>
  
  <p>here is my desired output:</p>
  
  <pre><code>index     col1     col2     col3
  0          3        8       np.nan
  1          2        2       np.nan
  2          2        np.nan  np.nan
  3          np.nan   1       1
  4          np.nan   6       6
  5          1        0       7
  6          2        1       1
  7          6        5       1
  8          np.nan   1       np.nan
  9          np.nan   6       1
  10         1        6       3
  11         6        3       4
  </code></pre>
  
  <p>I've been at this for hours. Anything is appreciated!</p>