From the full article:
Why not cut a Mini-SIM to fit a Micro-SIM slot?
Traditional Mini-SIM cards are slightly thicker than modern Micro- and Nano-SIMs, and so there is sometimes the risk that the cut SIM could damage the contacts on the phone’s SIM card reader. Many network operators today supply SIM cards with two pop-out sections so that the user can choose when they receive their new SIM whether to have Mini- or Micro-SIM; in these cases the whole card will be the same thickness as a Micro-SIM, so you do not have any risk, but you should NOT cut a standard Mini-SIM for phone designed to take a Micro-SIM. There is a secondary risk with cut SIMs, and this is that the cutting process can in some cases damage the metal chip and so render the SIM card useless.
Why not use an adapter to fit a smaller SIM into a larger slot?
If an adapter is used to insert a Nano-SIM into a Micro-slot or a Micro-SIM into a Mini-slot, there is the risk that the SIM card may be loose inside the adapter, in which case it can move during insertion and damage the SIM card contacts in the phone; or in some cases that the adapter assembly itself could be thicker than the SIM , which also may damage the contacts during insertion.
Good advice generally. Quite a few owners of the Nokia 808 PureView, for example, found that cutting a mini-SIM to micro-SIM format often caused issues, where exposed/cut SIM edges touched the metal of the card holder. While numerous smartphones (not just the aforementioned N8) have had issues with SIM slots being clogged up and/or damaged by nano or micro-SIMs in adapters, with ridges in thickness catching on SIM card sprung connectors.
In short, if in doubt, ask your carrier nicely to swap the SIM to the right size for your current phone. It can usually be done quickly and easily in a High Street shop and often without charge (I went through this twice in a week with Vodafone UK recently, without complaint).