Proof by induction on $\{1,\ldots,m\}$ instead of $\mathbb{N}$ I often see proofs, that claim to be by induction, but where the variable we induct on doesn't take value is $\mathbb{N}$ but only in some set $\{1,\ldots,m\}$.
Imagine for example that we have to prove an equality that encompasses $n$ variables on each side, where $n$ can only range through $n\in\{1,\ldots,m\}$  (imagine that for $n>m$ the function relating the variables on one of the sides isn't welldefined): For $n=1$ imagine that the equality is easy to prove by manipulating the variables algebraically and that for some $n\in\{1,\ldots,m\}$ we can also show the equation holds provided it holds for $n\in\{1,\ldots,n-1\}$. Then we have proved the equation for every $n\in\{1,\ldots,m\}$, but does this qualify as a proof by induction ?
(Correct me if I'm wrong: If the equation were indeed definable and true for every $n\in\mathbb{N}$ we could - although we are only interested in the case $n\in\{1,\ldots,m\}$ - "extend" it to $\mathbb{N}$ and then use "normal" induction to prove it holds for every $n\in \mathbb{N}$, since then it would also hold for $n\in\{1,\ldots,m\}$ .)

If the statement in question really does not "work" if $n>m$, then necessarily the induction step $n\to n+1$ at least somewhere uses that $n<m$. You may view this as actually proving by induction 
$$\tag1\forall n\in \mathbb N\colon (n>m\lor \phi(m,n))$$
That is, you first show
$$\tag2\phi(m,1)$$
(which of course implies $1>m\lor \phi(m,1)$) and then show
$$\tag3(n>m\lor \phi(m,n))\Rightarrow (n+1>m\lor \phi(m,n+1)),$$
where you can more or less ignore the trivial case $n\ge m$.
Of course $(2)$ and $(3)$ establish a perfectly valid proof of $(1)$ by induction on $n$.
