Stata is a program which is usually unforgiving of capitalization and spelling errors. However, as footnote 2 of this article in the Stata Journal points out, there is one instance where such mistakes are allowed, if not forgiven:
When the separate command was being written, one objection to its name was that even some people very good at spelling in English have difficulties spelling this word correctly. Thus, as you may have found, there is also an undocumented seperate command, which issues a peremptory message about your spelling, but then passes the instructions to separate anyway. Stata is not usually so forgiving.
As an aside, the proper spelling of Stata is “Stata” as the word is not an acronym.