tweets sent by text message (sms) are *reappearing* days, weeks later (canada)

Many friends of mine have had their SMS-sent tweets reappear in their timeline days or even a week later! To clarify, these are tweets that have already appeared before, and then they show up -again-, well after they were originally posted. It's rather disconcerting for them and for their followers, as you might imagine.

All of these friends are in Canada, on Canadian carriers.