Mysterious Riffle::rspec warning

Question

I have part of a code that looks like this:

horpatt = (* some code giving a List of n elements *);
verpatt = (* some code giving a List of n elements *);
tabpatt = Riffle[horpatt, verpatt] (* some ReplaceAll code *);

It is run several times with different values of the first two variables. Sometimes it produces this warning:

Riffle::rspec:
The third argument {2,-1,2} should be a positive integer or a list with three integers

which doesn't seem to be documented anywhere on the Internet. I try to see what might be wrong with the first two variables with the following code:

tabpatt = Check[
  Riffle[horpatt, verpatt] (* some code *),
  riffletrack = {horpatt, verpatt}; Abort[]
];

riffletrack gives me exact values of horpatt and verpatt that causes the warning. But when I try those values, the warning doesn't actually occur.

horpatt = {{"S", "Q", "U", "I", "Z"}, {"V", "U", "L", "G", "O"}, {Null, "O", Null, "G", "O"}, {"S", "T", "E", "E", "N"}, {Null, "H", Null, "D", "S"}};
verpatt = {{"S", "V", Null, "S", Null}, {"Q", "U", "O", "T", "H"}, {"U", "L", Null, "E", Null}, {"I", "G", "G", "E", "D"}, {"Z", "O", "O", "N", "S"}};
tabpatt = Riffle[horpatt, verpatt] /. Null -> _

Even more strangely,

Riffle[horpatt, verpatt, {2, -1, 2}] /. Null -> _

doesn't produce a warning, but a desired result that is identical to both of the following:

Riffle[horpatt, verpatt] /. Null -> _
Riffle[horpatt, verpatt, {2, 10, 2}] /. Null -> _

I'm not even sure what questions to ask. Does Mathematica automatically insert {2, -1, 2} as a third argument into Riffle? The warning seems to suggest that such argument doesn't work but why does it work? Why is there the warning in the first place? Should I always insert a proper third argument (akin to {2, 10, 2} above) to avoid this seemingly random warning? Is this a bug?

I'm using Mathematica 10.0.2.0 for Mac.

Update

I have actually put {2, 10, 2} as a third argument into my code:

tabpatt = Riffle[horpatt, verpatt, {2, 10, 2}] (* some ReplaceAll code *);

After many iterations, Mathematica eventually gives several instances of:

Riffle::rspec:
The third argument {2,10,2} should be a positive integer or a list with three integers

I'm pretty sure there's a bug in there somewhere.