A minyan starts to daven mincha before sheki’a with the intention to daven maariv after sheki’a. In the event, mincha extends beyond shki’a.
There are IMHO three possible cases:
They start but do not finish the quiet shmoneh esrai before shki’a.
They finish the quiet shmoneh esrai but not the repetition.
They finish the repetition but don’t finish the rest of davenning.
Is any of these situations “tartei desasrei” – two contradictory acts?
This question is related to but not identical to When is the earliest time you can Daven Maariv during the week?.