Possible bug in simulator

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Possible bug in simulator

I have been scratching my head for a few hours now, thinking i was wrong while having proof that i'm not.
The problem is the following:
          I load the .hdl and .tst file of my ALU into the simulator. I set the format to "Binary", and the view
          to "Compare".
          I check everything by using the "Single Step" button which is working like a charm for the first
          20 clicks, on click nr. 21 something ... weird happens.
          At this point i have the following:
                    Input: x[16]: 0000000000010001
                             y[16]: 0000000000000011
                             zx: 0
                             nx: 1
                             zy: 0
                             ny: 1
                             f: 0
                             no: 1

                    Output: out[16]: 1111111111111111
                                zr: 0
                                ng: 1

                    On program flow the following line is highlighted:
                    | 0000000000000000 | 1111111111111111 | 0 | 1 | 0 | 1 | 0 | 1 | 1111111111111111

          Now, with these inputs i should (if i'm correct) get this output:
                    out[16]: 0000000000010011
                    zr: 0
                    ng: 0

Is the simulator messing up somewhere?
Do note that this test file does not give any errors when doing a full check on my ALU.
My best guess is that it checks everything correctly, but the information it shows in the "Output", "Internal pins" and "Program flow" is wrong.
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Re: Possible bug in simulator

Very strange. Single stepping through the test using my ALU I never see the x[16] and y[16] inputs different than the x and y columns in the compare file.