Consonant gradation appears in the Finno-Ugric languages and for someone unused to it, it is easy to be tripped up by it. In Saame, consonant gradation is regular, but in Finnish it can appear downright arbitrary even years into studying the language. The worst culprit is the diabolical k, which can either disappear altogether, double, change into v, j, g, or not change at all.
On the other hand, there is also the problem of knowing when certain forms, like the genitive singular and nominative plural, turn the final i into -e.
These topics are discussed in further detail and contain lists of words that I have collected as I run across them in reading. Some of the words have been conjugated in their entirety and have descriptions of when to use them, possible synonyms in Finnish, example sentences, etc.