What should I not do with a magnet ?

The most important thing to avoid doing with a strong magnet, is letting it out of your sight.
This is for several reasons.
You do not want babies picking them up and putting them in their mouth, this is very bad !
You do not want to forget about them and walk away. They might stick to anything that passes nearby. (they love vacuum cleaners)
If you carelessly move another magnet nearby, the two may attract each other and collide with great violence. (I have shattered many Neodymium magnets this way)

Another thing NOT to do is underestimate it's power.
While carefully lowering a magnet towards a heavy steel plate, the plate may lift up to the magnet. Always immobilize a magnetic plate that a magnet will approach.

Another No-No depends on the strength of the magnets and how much you value your fingers. The crushing force between two magnets can be as strong as a hammer-blow.

If a second magnet or a piece of steel are "stuck hard" to a strong magnet, the most successful separation technique is to slide them apart. This may require using the edge of a non-magnetic tabletop to block one while the other is slid over the top.

Be careful that the separated magnets do not re-attract. Grab the second magnet or item before they separate.

Do not bring a magnet too near a color TV screen (CRT type). It can cause a strange shift in the displayed colors. If this happens, the screen needs to be "degaussed". Sometimes several degaussing passes are required.

If carrying a strong magnet from place to place. be aware of objects that may be attracted to it along the route.

And lastly, keep them clear of valuable magnetic recordings, such as VCR tapes, audio tapes, "floppy disks" , and credit cards.