In AutoCAD LT 2009 and earlier, when you're trying to hatch an area by picking points, it can be so frustrating to see the "valid boundary not found" error message. You had to go hunting around to see if you could find the gap, or increase your Hatch Gap Tolerance, which would create the hatch but make it non-associative.
In AutoCAD LT 2010, both those options have been significantly improved. (It still can't fix gaps in hatch boundaries for you, unfortunately.)
Using the Hatch Gap Tolerance (hiding in the expanded portion of the Hatch dialog) still results in a non-associative hatch. But, as I showed you in a previous post, this is no longer much of a problem when it comes to modifying the hatch later.
If you'd rather fix the gap than work around it, though, LT 2010 can help. When you pick a point that's not in an enclosed area, you now get a much more helpful message:
The little red circles go away after a Redraw or Regen.
No more hunting for tiny errors in a hatch boundary! (Although it can only find one or two gaps at a time, so if you have a very broken boundary you may have to run it a few times to find all the holes.)