I have a big magnetic sheet that my Raspberry Pi is sitting on. It seems fine, but are there any dangers?
As far as I'm aware, nothing in the Pi uses magnetic storage, so it shouldn't pose a problem. The SD card uses electric charge to store data (as does all flash media), and the ROM is either the same (if it can be reflashed) or it's burnt in at the factory and impervious to most external fields.
If you had a really strong magnet, it's possible you could induce some electric currents in the circuit as you moved the Pi in and out of the magnetic field, which could cause damage (if they are large enough currents), but I suspect the magnetic field would have to be so strong it would rip the Pi out of your hand first.
So in short, outside of a laboratory setting or an MRI machine, no magnet should cause any problems for the Pi.
I'm pretty sure a strong enough magnetic field would have a serious effect on the Broadcom SoC, especially the data it stores in ROM for booting up the Raspberry Pi from the SD card.
The question is; Is the magnetic field produced by your magnet strong enough to cause permanent damage? Probably not, but why would you want to risk it?
Probably you can have problems with the ethernet connector as it contains micro magnets inside. For more information check this: