HTR:

• set up two applications, app1 with dir1, app2 with dir2
• enable aliasing for app2
• try incremental syncing using app1, all works as expected
• now add a user to dir2, and set an alias for the user

Expected:

• app1 can still perform an incremental sync
• app2 falls back to a full sync (because alias events aren't supported during incremental sync)

Actual:

• both app1 and app2 fall back to full sync, the REST request for events to app1 produces the error:

HTTP/1.1 400 Bad Request

{
"message": "class com.atlassian.crowd.model.event.AliasEvent is not supported",
"reason": "ILLEGAL_ARGUMENT"
}

This is caused by mishandling alias events, and not filtering them when processing, even if they don't apply to the application that's making the request.