Now, I'm far from the expert on the ins and outs of the PPC. I don't know all the departments and the devices that have sprung up since my seven year absence.

I have done tense shifts in my writing before. Any author can do it, which is why it's important to have a good beta reader.

With that said, maybe Agents could experience something like a glitch in the Matrix. Either a repeat, or see everything around them physically shift (or swoosh or something) like good sci-fi special effects.

Sorry if people have already suggested these. I'm not in a reading-all-the-things mood.

~AW~
-I have a code en mai dose unt mai brain feelz fuzzy.

Or being tugged REALLY HARD at some random part of the Agent's body, causing him/her/it/whatever to end up splayed out on the floor in a disoriented mess.

I know that I've interpreted tense shifts as causing nausea and headaches, due to the stress of a different paradigm being imposed on the mind and brain. Anebrin is especially susceptible, with the handwave being that his faerie nature makes him more sensitive to such things.
Of course, that's just my two Cents.