Lucky dip

A somewhat embarrassing incident the other morning:
I often carry a small, pale blue backpack in my car with me. It contains my laptop, lunch, a few documents and various bits and bobs.
My pupil was asking about changing her address on her driving licence. Because my own driving licence was in my backpack I reached into the back seat to get my backpack. I opened the main compartment and rummaged through, looking for my appointments diary (which contained my licence). It didn't seem to be in the main compartment, which was worrying, so I looked in the smaller front compartment. Not there either so, getting quite anxious at the thought that I had mislaid my licence and my diary, I rummaged through the main compartment again. It must have been 20 seconds before I realised that I was looking in my pupil's pale blue backpack - not mine! I hastily zipped up her bag and spluttered an apology. She didn't really say much, but she certainly didn't seem bothered.
I still shudder at the thought of her wondering why I was looking through the contents of her bag. Hopefully not a mistake I'll make twice.