Routing the signal across the slot opening in the power plane
Every signal has a return current associated with it. That current will flow along the path of lowest impedance; it will "hug" the signal by flowing back through the adjacent plane (if possible). Note that bypass capacitors provide a way for the AC component of return current to jump between ground and power planes and can sometimes be more important for that purpose than they are for power integrity!

The slot will create an impedance discontinuity which will cause reflections and degrade signal integrity; an SI simulation will tell you if this is going to stop your design working reliably. And diverting the return current will open up the loop area of your signal, causing (potentially substantial) addition RFI emissions.

It's much less of an issue for a differential pair. There's a useful application note here.