A formula between $ signs, \(…\), $$, \[…\], or in a math fence is typeset while parsing. When its TeX doesn't parse, such as a \frac with one argument, a \begin with no \end, or a command the typesetter doesn't know, the page shows the TeX in a monospace font instead. Fix the TeX, or put it in code when it isn't math.
Incorrect
The ducks share the bread as \frac{b} each.
The ducks share the bread as $\frac{b}$ each.Correct
The ducks share the bread as each.
The ducks share the bread as $\frac{b}{d}$ each.