Let the decimal representation of $n$ be $\overline{1 a b c}$. If one of $a, b, c$ is $5, 6, 7$ or 8, then adding $n$ and $n+1$ requires a carry; if $b=9, c \neq 9 ; b \neq 9$ or $c \neq 9, a=9$, then adding $n$ and $n+1$ also requires a carry.
Therefore, $n$ must be one of the following forms:
$\overline{1 a b c}, \overline{1 a b 9}, \overline{1 a 99}, \overline{1999}$,

where $a, b, c \in\{0,1,2,3,4\}$. For such $n$, adding $n$ and $n+1$ does not require a carry. Therefore, there are $5^{3}+5^{2}+5+1=156$ pairs.
16. 2.3 * In $\{1000,1001, \cdots, 2000\}$, how many pairs of consecutive integers can be added without carrying over?