"chriscl" wrote:
Occasionally the = sign is not recognised as indicating
a formula, rather it appears as part of a text constant
i.e. typing =1+2 into a cell results in "=1+2" (without
quotes) appearing in the cell. In other cells it results in
"3" appearing as expected.

Previously, I mentioned that happens when you put a
space before "=". It also happens when the cell is
explicitly formatted as Text. See Format Cell Number.