Or get rid of all the approximations and do it exactly

For[rPos = 0, MAXRPOS <= 2 Pi, radiusInc += Pi/10,
  xPos = cellRadius Cos[rPos];
  yPos = cellRadius Sin[rPos];
  ];

That will avoid 20*ReallyReallyCloseToPiOverTen being either being slightly less than or slightly greater than ReallyReallyCloseToTwoPi and thus having 19 or 20 or 21 iterations and you can't figure out where the extra one is or the one that is missing went.

Then, if you absolutely can't resist the compulsion, you can stick in an N where you really really need a decimal approximation and that isn't being handled automatically by Mathematica hiding in the backgrount. Most things should be just as happy or even happier with 3*Pi/10 than with 0.942478.

Tracking down exactly what the root problem really really was might teach something and avoid that problem in the future.

A brain cell somewhere is making me think that years and years ago I stumbled across "unexpected behavior" with For. Maybe I'm remembering that the way I stumbled onto that was For[n=0, n<10, n=SomeFunctionOf[n], LoopBody] instead of n++ or n=n+constant. I think it was being more creative in the changing of the variable that didn't work as I expected. I told someone at the time and they reported it. Maybe there is an entry in the version logs that describe that being changed because I can't seem to reproduce that now. Or maybe I just can't remember the details correctly. But I can't get any of your examples to not work for me.

Inquiring minds want to know... what happens with a fresh start of Mathematica, no loading any other notebooks, etc, and exactly this

Print["does it work?"]

is typed into a cell and evaluated once, nothing more, nothing less.

(Trying to eliminate all the seemingly inessential detail)

If that doesn't display anything then I'd go hunting for initialization that might have gotten farbled and perhaps redirected some of your output, but you say that workbench is still telling you things, so that probably means some things haven't been completely broken.

rDistance = 5.4; radiusInc = N[Pi]/10; planeDepth = 0.5; zField = {};

For[rPos = 0, rPos <= (2 N[Pi]), rPos += radiusInc, zPotential = 0; Print@ToString@rPos; xPos = rDistance Cos[rPos]; yPos = rDistance Sin[rPos]; zPos = planeDepth; AppendTo[zField, {xPos, yPos, zPotential}]; Print@ToString@Length[zField]];

0

1

0.314159

2

0.628319

3

0.942478

4

1.25664

5

1.5708

6

1.88496

7

2.19911

8

2.51327

9

2.82743

10

3.14159

11

3.45575

12

3.76991

13

4.08407

14

4.39823

15

4.71239

16

5.02655

17

5.34071

18

5.65487

19

5.96903

20

6.28319

21

Try inserting Print statements in between some of the lines

For[rPos = 0, rPos<=(2 N[Pi]), rPos += radiusInc,
    zPotential = 0;
    Print@ToString@rPos;
    xPos = rDistance Cos[rPos];
    yPos = rDistance Sin[rPos];
    Print@ToString@{xPos,yPos};
    zPos = planeDepth;
    AppendTo[zField, {xPos,yPos, zPos, zPotential}];     
];