You can turn off strictNullChecks, and there's also noUncheckedIndexAccess which unfortunately isn't enabled by default even in strict mode. You can also introduce flaws via @ts-ignore, casting, etc.
Still, typescript equips you to catch these errors, even if you can technically circumvent it. In practice it can be nearly bullet-proof if you follow good practices.
Aside from explicitly turning off null safety and tricky use of casting, an easy example is interfacing with JS.
If you're using either a library that wasn't written in pure TS (maybe JS or JS with .d.ts) or interacting with some unconverted JS from your own codebase, you can easily pass a null through entirely by accident. The problem really stems from the JS end of things, but 9 times out of 10 you're going to be touching JS at _some_ level when using TS so I think it's fair to point out this gap.
