Why 503 errors can happen?

[mihalt]

After running SkypeEventLoop on production very rare, but we have

SkypeApiException
('503 response from POST https://azeus1-client-s.gateway.messenger.live.com/v1/users/ME/endpoints/SELF/subscriptions/0/poll
', <Response [503]>)

Also were urls like:
https://azeus1-client-s.gateway.messenger.live.com/v1/users/ME/conversations/ALL/messages/1730639857828/ack
https://azeus1-client-s.gateway.messenger.live.com/v1/users/ME/conversations/8:live:12345

@Terrance do you have any ideas why could it be? Are maybe some 50* errors from skype is ok to see? How do you recommend to catch and investigate errors like this in my logging system?