Menu

The menu system has a very old history in Drupal. The system as we knew it, began its life in 4.5 (2004 September, actually) when JonBob added back menu caching. As not all menu items were cacheable some code defined parts were in cache, some parts were not. To make it customizable, again some information was kept in a normalized database table when menu module was enabled. The complex structure kept every menu link for every page load. This was a performance and a memory problem. Also, the primary index was the path.

Here's a status report on the menu-related issues for 6.x that I'm aware of - please add to/update this list:

RTBC in the queue:

Adding new node link with the same path will hide other links to the same node
http://drupal.org/node/179578

Tabs rendering broken if non-existing item is in path
http://drupal.org/node/179474

arbitrary number of arguments in router path are not supported
http://drupal.org/node/177488

patch code needs review:

Ugly breadcrumbs
http://drupal.org/node/176748

Inconsistent titles and breadcrumbs at various levels in aggregator