Apple is pushing out a public beta of OS X El Cap (10.11) today for folks. There's always bugs in beta software (otherwise it'd be a final release, right?), so expect to see them if you play with these new bits.
How does Acorn run on 10.11? Pretty good! We've found some issues though, and we've got them fixed in the latest builds (which will be 4.5.6 eventually). So if you're playing with 10.11, please grab that version of Acorn.
And of course- if you find any issues we don't know about, let us know!