I think this is a relatively recent regression. `\ ` at the start of a line normally gets treated as a space:
> pandoc
one
\ two
three
<p>one</p>
<p> two</p>
<p>three</p>
But if it's in a table, it gets preserved now. HTML below:
> pandoc
one | one
----|---
two | two
\ | three
<table>
<thead>
<tr class="header">
<th>one</th>
<th>one</th>
</tr>
</thead>
<tbody>
<tr class="odd">
<td>two</td>
<td>two</td>
</tr>
<tr class="even">
<td>\</td>
<td>three</td>
</tr>
</tbody>
</table>
Native output looks like this:
[Table [] [AlignDefault,AlignDefault] [0.0,0.0]
[[Plain [Str "one"]]
,[Plain [Str "one"]]]
[[[Plain [Str "two"]]
,[Plain [Str "two"]]]
,[[Plain [Str "\\"]]
,[Plain [Str "three"]]]]]
Versus this for the non-table version:
[Para [Str "one"]
,Para [Str "\160two"]
,Para [Str "three"]]
This has only started recently. I noticed it today in a document I last processed in October 2017.