If you're working on a phpBB install set to development and you have PHP set to show notice-level errors, you'll see an error about a missing post_id.

Steps to reproduce
1) Go to MCP page for a topic while logged in as a moderator or admin
2) Select one or more posts
3) Select the "Delete posts" action from the select at the bottom of the page
4) Confirm the deletion action

Expected
Success message.

Actual
Error message: ContextErrorException in mcp_main.php line 1040: Notice: Undefined index: post_id

Issue is a missing post_id key in the array of data passed to the moderator log.