There is a longstanding debate about how to appropriately model the combinability of affixes, especially English suffixes. One widely accepted principle is the notion of so-called selectional restrictions, i.e. grammatical requirements of particular affixes. For example, the suffix -ness can only combine with adjectival bases. Hay (2002) proposed a psycholinguistic approach to affix ordering now known as Complexity-Based Ordering (CBO), which claims that affix order is determined by the parsability of the affixes, i.e. more separable affixes can appear only outside of less separable affixes. Hay shows that this principle accounts for why many grammatical affix combinations are unattested. CBO has since been supported by research of derivational affixes (English prefixes, English suffixes and Russian suffixes). However, as a processing model, CBO should apply very broadly, and in this paper, I discuss some difficulties of reconciling CBO with inflectional affixes. I also examine combinations of Russian prefixes - which have some properties typical of inflection - and show, surprisingly, that they can be ordered with a significantly low number of cycles, as CBO predicts. I discuss alternatives to CBO that explain this phenomenon, and suggest future research to distinguish them.