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Butterfly Bend
This is a pretty decent bend that is nothing more than a Butterfly Loop with the loop cut.  It's not completely symmetric, but it has good security and is pretty easy to tie and untie.

                        Evil Impostor

Evil Imposter ABoK 1408/1409 Ashley Bend ABoK 1452
Evil Impostor of Ashley Bend & Butterfly Bend  (ABoK #1408/1409) Ashley Bend (ABoK #1452)

Both the Butterfly Bend and the jam-prone Ashley Bend* can be mistakenly tied to form something that looks like the intended knot, but has poor security properties and can roll apart under certain loads in certain conditions.  What  makes this more dangerous is that the negative properties of this evil impostor are not usually apparent when setting the bend.  

This evil or malignant imposter bend is discussed by Ashley in his Book of Knots as entry #1409 and as its more stable shifted form under entry #1408.  The only difference between the two is that the more stable form has its free ends rotated or twisted relative to one another in the opposite direction from the insecure version. 

Although the danger of tying this imposter is greater in the attempted Ashley Bend, one can imagine the "b" shape rotated counterclockwise, revealing that this same path of error for the Butterfly Bend is at least possible.  The general knot form of the half hitch loop is another possible evil impostor for the Butterfly Bend and the Butterfly Loop, especially with some alternative tying methods for the loop.

If you feel that you may make such a mistake for these bends, the Zeppelin Bend is an excellent alternative, which has no serious impostor risk when tied with the "b" and "q" method, and may be immune to evil impostors regardless of method.  The issue does not seem to affect the two common means of tying the Butterfly Loop shown here.

*Clifford Ashley did not name this bend.  He merely listed it in his book as entry #1452.  He remarked that it was more complicated than could be wished.