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Dr Darren of Webel IT Australia says:
This basically describes a Mathematica "newbie" mistake, which doesn't mean a non-newbie can't overlook it, as I have sometimes. It can make it really hard to find self-caused bugs. I thought to share it in public as it might help someone else.Note to self: Don't do this ever again:
fRows[nRows_Integer] := Module[
{ },
...
Table[ fThing[iRow], {iRow, 1, nRows}]
];
That "loop variable" iRow that was used within the Module will carry a value exposed outside of the Module, because it was not declared within the scope of the Module (and won't be uniquely locally decorated as iRow$uniqueId). If you use iRow in an Expression elsewhere, it can cause very strange side-effects, especially if you have a function with Head iRow, such as trying to invoke 7 on something!
Always declare a loop variable used in Table within a Module explicitly!
fRows[nRows_Integer] := Module[
{iRow},
...
Table[ fThing[iRow], {iRow, 1, nRows}]
];