EDIT: In my current game, I invaded an AI homeworld and it was EMPTY, except for the Starting Colony and Starport. The AI was trying to build a Basic Factory, and it was stuck at 1 Week left, probably due to this bug. This is unacceptable, as the AI has no way to "manually" work around this bug.

If you have a colony improvement in your build queue, lets say "Basic Lab" for example, and then you discover the tech for a superior version (let's say "Xeno Lab") before the old one reaches the top of the build queue, then the obsolete building will get stuck at "1 Week Left" when it reaches the top of the build queue. For example:

build queue
-----------------
Factory (3 Weeks)
Factory (6 Weeks)
Basic Lab (8 Weeks)

then I discover Xeno Labs...

build queue
-----------------
Factory (2 Weeks)
Factory (5 Weeks)
Basic Lab (7 Weeks)

after 5 more turns...

build queue
-----------------
Basic Lab (2 Weeks)

then you can let it run for any number of turns. You end with this state and can only get past it by manually deleting the obsolete building:

build queue
------------------
Basic Lab (1 Week) <- it will be stuck like this forever