# Jerky Plot features display in exported GIF

aa1 := Show[Plot[{Tan[x], 0}, {x, 0, t},
Mesh -> {{0, Pi/8, Pi/4, 3 Pi/8, Pi/2, 5 Pi/8, 3 Pi/4, 7 Pi/8, Pi,
9 Pi/8, 5 Pi/4, 11 Pi/8, 3 Pi/2, 13 Pi/8, 7 Pi/4, 15 Pi/8,
2 Pi, 17 Pi/8}},

MeshStyle -> {Directive[PointSize[Medium], Blue]}],
PlotRange -> {{0, 2 Pi}, {-1.5, 1.5}},
Ticks -> {{0, Pi/8, Pi/4, 3 Pi/8, Pi/2, 5 Pi/8, 3 Pi/4, 7 Pi/8, Pi,
9 Pi/8, 5 Pi/4, 11 Pi/8, 3 Pi/2, 13 Pi/8, 7 Pi/4, 15 Pi/8,
2 Pi, 17 Pi/8}, {-2, -1, -1/2, 0, 1/2, 1, 2}}
];
bb1 = ParallelTable[aa1, {t, 0.1, 17 Pi/8, 0.05}];
Export["grafica_000.gif", bb1]


The code produces the following image, but how do I avoid movement on $Pi/2$

I changed a bit settings of your code so you have more control over the display of animation. The key fix is using Exclusions->None. You can also use 'Exclusions->Automatic'. In future please pay attention to your question titles (you had "A problem with ParallelTable") --- obviously ParallelTable itself got nothing to do with your issue, as it is of graphical nature.

meti={0,Pi/8,Pi/4,3 Pi/8,Pi/2,5 Pi/8,3 Pi/4,7 Pi/8,Pi,9 Pi/8,
5 Pi/4,11 Pi/8,3 Pi/2,13 Pi/8,7 Pi/4,15 Pi/8,2 Pi,17 Pi/8};

aa1[t_]:=
Plot[
{Tan[x],0},{x,0,t},
Mesh->{meti},
MeshStyle->{Directive[PointSize[Medium],Blue]},
PlotRange->{{0,2 Pi},{-1.5,1.5}},
Ticks->{meti,{-2,-1,-1/2,0,1/2,1,2}},
PlotStyle->{Blue,Directive[Red,Thick,Dashed]},
Exclusions->None,
ImageSize->500
]

bb1 = ParallelTable[aa1[t], {t, 0.1, 17 Pi/8, 0.05}];
Export["grafica_000.gif", bb1, "AnimationRepetitions" -> Infinity]