Floating point errors can cause incorrect code to generate. This depends on the language, but as an example we noticed that encoding latitude 1, longitude 1 produces an X for the 11th digit.

The 10 digit code latitude and longitude ranges are the same - 1.0 to 1.000125. In the 11 digit sub-grid, the characters are assigned as:

So for latitude 1.0, longitude 1.0 we want to get 2. But we are getting X.

This seems to be a problem in the encodeGrid functions where we try to work out which column the longitude is in by getting the remainder of dividing the longitude by the grid size (0.000125).

You can check this in Javascript by doing:
1.0 % 0.000125

It should be zero, but in my browser I'm seeing 0.00012499999999997919 which is nonsense.

Instead of directly comparing values in that function, we may need to use "close enough" comparisons, where if numbers are within 1E-10 of each other that's good enough.

Another strategy is instead of doing the remainder of 181.5 % 0.000125, we ditch the degrees and use 0.5 % 0.000125.

The solution in each case is probably language dependent.