Redmine

1. Work on some Trac instance, making use of repository keyword references
   1. Create a pair of tickets, numbered A1 and B1
   2. Commit a change summary for revision X like "fixes #A1"
   3. Commit a change summary for revision Y like "fixes #B1"
2. Work on some Redmine instance until its ticket count surpasses A1 and B1
3. Import the Trac instance into Redmine
   1. Note that Trac's tickets are renumbered to A2 and B2
   2. Note that when this project is connected to its repository, old change summaries reference incorrect tickets.

I guess the root of this is that given a sufficiently large set of tickets and commits, there exists a set such that A2 == B1.

A simple solution would be to disable issue references from before the time a project was imported.