This page runs two operations for ProjectA and, if they all work, up to three for ProjectB. Each of ProjectA's operations has about a 30% chance of failing, as do the first two of ProjectB's. If we reach the third ProjectB operation, we force it to fail.
ProjectA and ProjectB have both registered
onerror handlers (playing nicely) and
both handle errors they see, suppressing the
browser's default handling of them by returning
true if the error is theirs.
Refresh the page enough that you see both ProjectA an dProjectB fail.