The active perfect stem conjugation in Latin resembles the conjugation of esse a lot, but I recently learned that it is likely to be a coincidence. However, the active perfect indicative forms do not resemble esse — or any other Latin conjugation.
What is the origin of the personal endings (-i, -isti, -it, -imus, -istis, -erunt/-ere)? Do they come from the same place (PIE perfect, for example), or is it a collection of elements from a number of sources? There are familiar elements (-t in third person singular, -mus in first person plural, -tis in second person plural, -nt in third person plural), but the combination of all six is unlike other personal endings. There are also elements that are totally unexpected based on other Latin conjugation.