After testing my theory out tonight in a 10 man raid with a Demonology warlock I can confirm that every time I cast a rejuvenation when Demonic Pact was up I failed to refresh the spell when that buff wore off and the previous rejuv was still present on my target. The error message was "A more powerful spell is already active" and the combat log contained the text "Demonic Pact is refreshed on Niniel" before it happened.
I see what you did there Mr Warlock!
Unless somehow proven wrong I hereby state that Demonology warlocks with that talent are not the best friends of druid tank healers.
If there is no Demonology Warlock in your party/raid the other candidates for this error message is a change in any buff to your spirit or spell power. Priests with their spirit buff or a shaman with a spell power totem are likely. Priests and shamans are much better though since the shaman can keep the totem up at all time and the buff from the priest should be constant.
I don't mind spell power buffs but the problem with the warlock talent is that it's random and inconsistent. I would prefer to have control over when my spell power change so that I can plan rotations accordingly and refresh them at will. DoT and HoT rotations suffer from this effect when we can't refresh. It's more important to keep the flow of damage or healing layered and constant than loose the rhythm and stacks due to this error message.
I really hope the mechanics for this is changed so that we can override the stronger spell at will.