System Information
OS X 10.6.8
NVIDIA GeForce 320M

Blender Version
Broken: 2.71 9337574
Worked: Never, as far as I know.

Short description of error
In Edit Mode sometimes pressing the L key will Select Linked vertices. Sometimes it does not.
Because the behavior is inconsistent I have made a habit of clicking a vertex first and then Selecting Linked.
But it really shouldn't be this way. While preparing some class curriculum for beginners of Blender I realized that I would not be able to explain to a student why Select Linked behaves as it does. This got me thinking it could actually be a bug.

There is also a problem with the selection overflowing into vertices which are NOT linked. I'm not sure if these two issues are related but it's worth mentioning them together since they might be. The overflow bug is hard to replicate consistently, however I was able to replicate it in this minute-long unedited screencast. See the .mov attached. I am also including a .txt file of the console output that goes along with that screencast. The selection overflow occurs 38 seconds into the video. You can see I start with the default scene and try to replicate the bug four times, then finally succeed on the fifth. Finally I select just one piece to demonstrate it is in fact a separate piece.

For some reason Blender is not always succeeding at selecting linked vertices. It doesn't seem to always know which are connected and which are not, which is strange because geometrically speaking it's pretty black and white. Either a piece is connected or it's not. Maybe Select Linked uses some viewport-based approximation to calculate and it's lacking in accuracy.

Exact steps for others to reproduce the error
For the bug where Select Linked fails:

1. Starting from the default scene, Tab into Edit Mode, then Box Select or Shift-RClick to select four vertices that form a face of the cube (do not use face select or it will succeed)
2. Perform an operation on that face, such as Grab, Rotate, Scale, or Extrude.
3. Press the L key. Select Linked fails.

For the bug where Select Linked overflows:
This is a tough one because it doesn't happen every time, but you can see from my video that it does happen frequently enough to observe if you try several times.

1. Starting from the default scene, Tab into Edit Mode and Shift D to Duplicate the cube mesh. Move it 2 Blender Units on any axis (I used Y).
2. Rotate and/or zoom the viewport arbitrarily.
3. Press the L key. Sometimes it will select the unconnected cube mesh as well as the connected one.

Screencast:

Console Output: