My formulas are changing automatically and I can't figure out how to turn off
the feature. Example;

Columns C thru K contain numeric values. Column M contains a the formula
SUM(Cx:Ex) where x is the row #. As I'm keying data in cells C5, D5 & E5
everything is fine. When I key a number into F5 however the formula
automatically changes to include column F (i.e. SUM(C5:F5)). Excel makes the
assumption that just because there's a number in F5 that it should be
included in the formula. I've gone into ToolsOptionsError Checking and
played with the settings even so far as disabling 'background error
checking'. Now Excel changes my formula, it just doesn't call my attention
to it with a colored triangle in the upper left of the cell.

I know I'm missing something, any idea what?