I am currently reading Shalika's article "Representation of the two by two unimodular group over local fields" and variuos other related articles, which deal with the classification of complex representation of reductive groups over local rings. It is cumbersome, that some authors consider only local rings with residue fields of characteric $p \neq 2$.

Why is $p=2$ special here?

Perhaps some words about the strategy, which one should use - at least from my perspective: Consider a finite extension $K$ of $\mathbb{Q}_p$. Let $\mathfrak{o}$ be the ring of integers in $K$ and $\mathscr{p}$ its maximal ideal. We wnat to classify all representation of $$\mathrm{GL}_2( \mathfrak{o}) = \lim\limits_{n} \mathrm{GL}_2( \mathfrak{o}/\mathfrak{p}^n).$$ We proceed by induction over $n$:

1. Classify all representation of $\mathrm{GL}_2(\mathbb{F}_q)$, where $\mathbb{F}_q$ is the residue field.

2. Use Mackey's formalism for the group extension (non split) $$ 0 \rightarrow M_{2\times2}(\mathbb{F}_q) \rightarrow \mathrm{GL}_2( \mathfrak{o}/\mathfrak{p}^n) \rightarrow \mathrm{GL}_2( \mathfrak{o}/\mathfrak{p}^{n-1}) \rightarrow 0.$$

Apparently the difficulties do already arise in step 1, since Piatesko-Shapiro in his lecture "Complex representations of $\mathrm{GL}_2$ over a finite field" only considers characteristic $\neq 2$.