How to reproduce:
1. Download attached project file and open "testScene" scene
2. Enter Play Mode. Notice Console log:

Plays at 8 (a debug statement from the script to say it's calling test anim at frame 8)
Starts at 8 (the first animation event)
End at 19 (the second animation event) (this makes sense, the animation lasts 12 frames (frames 8 through 19 together). However, if you move the second event over to 0:12 something odd happens)

3. Move test animation event from 11 to 12
4. Enter Play Mode. Because you moved it one over, you expect it to finish on frame 20. You get the following messages:

Plays at 8 (a debug statement from the script to say it's calling test animation at frame 8)
Starts at 8 (the first animation event)
End at 21 (the second animation event)

Actual result: It ends on frame 21, meaning an extra frame happened here. Our 13 frame animation has lasted 14 frames and that is the core of the problem.

Reproduced with: 5.6.5f1, 2017.1.3p1, 2017.2.2f1, 2017.3.1p2, 2018.1.0b9, 2018.2.0a2