It really is great to have fun with our budgies and get to know all of their quirks that makes them unique!
While it's not the norm, females can sometimes lay just the one odd egg even when we are taking measures to keep them from laying.
This can happen just as they first come into condition.
Personally, I never had a female budgie to do this but I do have a blue masked lovebird that lays the one odd egg at times. I have been keeping track of her cycle and this year she laid two eggs. One on Easter day and the second one on the beginning of October. Even though in her case I can't look at the cere for clues, I've come to the conclusion these events are linked to her coming into condition and this is just the way her body reacts to coming into condition and this is not considered a chronic egg laying case.