I have discovered a scenario that I believe is not by design.
When using a table in a RadGrid GridTemplateColumn, the <td> tags render with a bottom border. This behavior appears similar to that when a browser renders 'default' borders for a table that has not specified otherwise. The exception, in the scenario mentioned this occurs only for the bottom border of the cells and only occurs in alternating rows.
Reproducing should be as simple as adding any table to a GridTemplateColumn and setting alternating row color to white (for visibility).
CSS class to explicitly define cell borders with !important declaration(s) to override RadGrid style sheet.