An example that only involves basic algebra:

```
In [70]: a = 1/(x + 1) + 1/x

In [71]: (a - cancel(a)).evalf(subs={x:1})
Out[71]: 0.e-125
```

This actually can show a disadvantage of using `evalf(subs={x:a})` instead of `subs(x, a).evalf()` (`(a - cancel(a)).subs(x, 1) gives exactly 0).