A long tail of subtle event and reentrancy bugs, where an event loop wasn't getting an event it needed (leading to dropped or mishandled OS events), or a function that doesn't expect to need to be reentrant was suddenly reentrant.

Most of these could be worked around, but some of them are unreasonably difficult given OS event apis.

We replaced it with just making the calls that might need to be asynchronous use `async`/`await`.