An interesting tweet from LeoLabs is warning of a potential collision between two objects in a relatively low earth orbit (991Km):
They are predicted to pass within 25 metres of each other, with a probability of them hitting each other between 1 and 20%. So whilst still unlikely, there is still a pretty good chance of them colliding.
The radar cross sections are 8.6m2 and 2.2m2, so when they get within 25m of each other, they are going to be fairly close.
And if they do hit, the energies involved will be huge, resulting in both being broken up into thousands of pieces as the energy released will be the equivalent of hundreds of Kilograms of TNT (rough estimate).
Since the collision is in low earth orbit where there are a large number of satellites and with debris flying off in multiple directions with large energies, there is a risk that the debris may strike other objects which could be satellites or even just other debris. This would result in even more debris.
Of course there is the potential for there to be a runaway chain reaction, which is known as the Kessler Syndrome.
To learn more about that, there is an excellent video on YouTube from Curious Droid which is well worth watching (and his other videos too).
The chances of this happening is relatively low, but it could happen.