http://boincstats.com/stats/boinc_user_graph.php?pr=bo&id=1514450

Can anyone explain the cause of the weird stats on the 25th of April?

It's as if milkyway@home project simply vanished from existence and the stats took it into account giving me:

Negative(???) 40-something-thousand credit (and associated world position change).

Sure it 'came back', and my world position went back up a few hundred thousand the next day, but now I have ridiculous looking graphs and one that wont display (illegal pie plot) at all.

There was a time recently when the project was offline for a while but I don't recall the exact date. I suspect they might be related, if this is the case, are these kinds of stat bugs common?

I would not call it common but it does happen. Usually the cause is a split CPID. When it recombines you get the positive spike.

Whenever you join a new project you get a new CPID, in this case it normally changes to your existing CPID before anyone notices.

If you change your email address you will get a new CPID, it will propagate through all of your projects but may take several days.

It's happened once again so I must be unlucky. I haven't done anything to the hosts or changed any boinc related settings this second time (I may not have the first time either, but was doing alot of different stuff with the hosts a few days ago when the first problem happened.)