Grace Notes and Trills Behaving Oddly
I was working on transcribing a piece that has a grace note lead-in to a trill, as shown here: 
Now, while producing it in Musescore is easy, the playback is broken. The trill doesn't respect the grace notes at all; the trill, and the grace notes, start on the beat and play on top of each other. I can't imagine that's intended behavior. Reproduction is as simple as placing a note, adding a trill ornament, and adding a grace note.
Comments
Grace notes in MuseScore playback according the most common standard, which is to put them on the beat. There are indeed cases where this isn't correct interpretation, and this is one of them, but anyhow, we don't currently suportt before-the-beat playback, And hence you get overlap. Someday we may add support for before-the-beat playback; it's definitely been requested.
In reply to Grace notes in MuseScore… by Marc Sabatella
In this case it's not even a matter of before or after-the-beat playback; the trill itself doesn't wait for the grace notes to finish, period. The grace notes start on the beat, and so does the trill.
In reply to In this case it's not even a… by LuuBluum
Sure. The algorithm could indeed be tweaked to have the trill wait.