1)  [   1    Computers can't think], because [  2   computers are machines], and [   3   machines can't think].
                  2   +   3       Valid, no fallacy.  Note: to have [2] as a reason for [3], or [3] as a
                        |                 reason for [2], is seriously wrong!!!
                       1

2) [   1   No one has supplied any clear proof that the U.S. is covertly supplying arms to rebel forces in Columbia].  We may thus conclude that [   2   the dissidents in that country are receiving no military assistance from the U.S.]

        1 -->   2      Invalid, weak

IG:  The mere lack of proof that that the U.S. is supplying arms is inadequate for concluding that the U.S. isn't supplying arms.  After all, major efforts are often made to keep arms shipments secret!

Note: this doesn't provide any basis for concluding that the U.S. either is or is not supplying arms to those dissidents.  If all we have is lack of proof, then we just don't know!

3) Environmentalists keep telling us that [  1  coal-fired electric generating plants cause acid rain], but [  not 1   they're just wrong].  [  not 1   They aren't the cause], and the reason is plain as can be:
[  2  no one has yet proved that burning coal produces acid rain].

   2  -->  not 1       Invalid, weak.  Note that the 1st sentence is basically background information; without that sentence, you wouldn't know what the "they's" refer to.

IG:  Mere failure to prove that burning coal does cause acid rain is inadequate for concluding that it doesn't cause acid rain!
 

4) [  1  Mrs. Johnston's suggestions about how parents can deal with problem children are hardly worth taking seriously]. After all, [  2   she's never had and children of her own, you know].

      2 -->  1       Invalid, weak.  "After all" as an inference indicator is a bit of a judgment call, but it does function about like a "because."

AH.  The reason merely gives personal information (mildly critical) about Mrs. Johnston.  It doesn't even say anything about what her suggestions were, nor anything at all about their flaws or weaknesses!

5) [  1  It's not a good idea for Congress to consult Pentagon officials for advice about the military spending bill]. [   1   They shouldn't consult them] because, [  2  as members of the armed forces, Pentagon officials are going to try to get as much money as possible into the defense budget].

    2 -->  1       Invalid, weak

AH.  The reason basically says that Pentagon officials are greedy - definitely criticizing them personally.  But Pentagon officials do know a good bit about military matters, and their advice might be worthwhile despite their greed.  Nothing at all is said about what their advice is, nor about why it is or isn't good advice.

6)  [  1   Laws against discrimination are ridiculous], for [  2  life would be impossible without discrimination].  [  3  We have to discriminate between tastes to select good food; between pleasant and unpleasant sounds in selecting our music; between safety and danger when driving a car].
[   2   You can't live without discrimination]!

   3  -->  2  -->  1       The "3 --> 2" part: either valid or strong
                                   The "2 --> 1" part: invalid, weak, fallacious

EQ: The meaning of "discrimination" shifts from "making careful judgments" (in statements 2 & 3} to "prejudice or bias" (racial, gender, etc.) in the conclusion.
 

7) [  1  It is your duty to do what is right], hence [  2  it is your duty to reject good advice], because [ you certainly have the right to do so].

    1  +  3      Invalid, weak   (1 as a reason for 3, or 3 as a reason for 1, is seriously wrong!)
         |
        2

EQ: The meaning of "right" shifts from "what you morally ought to do" (in 1) to "privilege" or "freedom" (in 3).