My table was defined something like this...
Inserting values that I thought should have worked always failed. This is the kind of INSERT statement I was running...
...and the result - always '0.999'.

The problem turned out to be with the DECIMAL(3,3) line. I didn't read the documentation fully before jumping in so what I ended up creating was a field that had 3 digits with 3 digits after the decimal point. So naturally MySQL interpreted that as numbers that would be in the range of .999 to .000. It appears that storing a number larger than 0.999 ends up storing 0.999 all the time i.e. the maximum value this field can store.
What I wanted of course was a field that had 3 digits BEFORE the decimal point and 3 digits AFTER the decimal point. The solution - simply change the column definition to DECIMAL(6, 3). In this definition, the 6 was the total length of the field and the 3 was how many digits to allow after the decimal point. The range that way is 999.999 to 000.000.
The table definition now becomes...
Running the same INSERT statement as before, the data was now stored correctly...

So it pays to read the documentation before assuming to know how a particular product to works. In my case I got caught off guard assuming I knew how MySQL DECIMAL worked and ended up wasting time resolving this issue that could have been spent more productively.
-i