A few people have mentioned not flying across an instrument approach, and while that's obviously a good idea, if you've only got a basic PPL nobody tells you anything about instrument flying.
When I was learning ATC would sometimes say 'I have have traffic on an instrument approach' which meant absolutely nothing to me - if they'd said on right base I would have known where the plane was. I discussed this with my instructor and he ran through instrument procedures with me so I could have a basic appreciation of what was going on.
However, you could do your 45 hours, get your license, and not even know what an ILS was, so unless ATC keep you out of the way you wouldn't have a clue that you were interfering with instrument traffic.