Trendline Formula Wrong - What mistake did I make?

I see no evidence in your attached Excel file to indicate any error in the trendlines.

But let me guess: you used the displayed precision of the coefficients in formulas, and you found that your calculated data points are not close to the apparent trendline data points in some cases.

The mistake would be: using the imprecise coefficients.

I suggest that either you format the trendline label to show the coefficients with greater precision (Scientific with 14 decimal places, in general; Number with 17 decimal places, for your current formulas), or you use LINEST to calculate the coefficients directly in the worksheet.

Actually, when I tried the latter for the AUTOBRAKE1 data (B8:K8), I could not duplicate any "mistake" even with the less precise coefficients.

Nevertheless, see the formulas and coefficients in row 9 and row 10 as a demonstration of the principle.

PS, for completeness.... Another common mistake is to use the trendline formulas in a Line Chart, instead of an XY Scatter chart. You __did__ use the latter; good. But to explain: a Line Chart always uses x=1,2,3,... instead of the values that we display on the x-axis. Consequently, the Line Chart trendline coefficients are wrong if the actual x-values are different (e.g. 10,20,30,... in your case).