trend/linest regression error

This is a fairly simple section of a spreadsheet I use to find a point on a line defined by two points. I use the =TREND() function for this. I had never noticed this before (maybe because I have been using version 2002 until about a week ago), but on occasion, the TREND() function will return an incorrect result. The behavior I am describing is illustrated in the attached spreadsheet.

Explanation of spreadsheet.

input data for the functions is in columns A through E. In order to understand what the TREND function was doing, I calculated the slope and intercept using both LINEST() and the separate SLOPE and INTERCEPT functions. I observe that LINEST() appears to force slope=0 for some reason. I'm guessing that it is somehow detecting "co-linearity" and consequently declaring the slope 0. I know when I first read that MS had introduced this co-linearity test into the LINEST() function that I was concerned that it would occasionally falsely/erroneously detect co-linearity.

I'm not sure that I really need anyone to propose a solution -- the obvious solution is to use the algebraic function in column Q or use the SLOPE()/INTERCEPT() functions instead of TREND()/LINEST() in this case. What I'm interested in is 1) does this only occur in this one installation of Excel (I will test this tomorrow at work on both 2007 and 2002)? 2) Has anyone else experienced situations where these co-linearity tests have erroneously detected co-linearity and forced needed coefficients to 0?