Number of optimal solutions for Longest Common Subsequence (Substring) problem
Consider the following two strings:
$$
s_1 = (abc)^n, s_2 = (bac)^n.
$$
You can show that the longest common substrings of $s_1,s_2$ have length $2n$. There are at least $2^n$ of these: $\sigma_1 c \sigma_2 c \ldots \sigma_n c$, where $\sigma_i \in \{a,b\}$.