I've found quite a few things, especially on spamcans I used to rent, but the only ones that were dangerous were (a) bare landing light wires hanging around the engine compartment (Tomahawk) which could have shorted to the airframe resulting in a fuel fire/explosion (which the instructor wasn't bothered about and simply told me to not use the landing light - I walked out of the school after that**) and (b) two occassions where the tanks were found totally empty and wildly differing from the fuel in the tech log, either due to overnight fuel theft or due to previous pilot(s) having got very very lucky... Consequently, the subsequent G-OMAR accident was no suprise to me at all.
** One wonders what sort of stuff would be found in flying school spamcans on which the preflight does not include looking under the engine covers (i.e. most of them).
I also once found a slightly flat tyre - this needs to be taken seriously because if it is a recent slow puncture, the tyre could be totally deflated by the time one lands. In this case, the cause was a tiny puncture in the side of both the tyre and the inner tube; god knows how that happened...