SMPS transformer secondary voltage drift
To add an example to the hints in the comments, here is a basic way to do it:

The top plot shows the current in the secondary, and the two branches vary differently, yet the bottom plot shows how both the outputs are affected. The outputs try to stay at 12 V and 5 V due to the common feedback, but the outputs cannot be isolated since they are physically on the same transformer. The slight variations in the &quot;stable&quot; portion are also slightly afected.