What happens if I don't cancel a Context?

If you fail to cancel the context, the goroutine that WithCancel or WithTimeout created will be retained in memory indefinitely (until the program shuts down), causing a memory leak. If you do this a lot, your memory will balloon significantly. It's best practice to use a defer cancel() immediately after calling WithCancel() or WithTimeout()

If you use WithCancel, the goroutine will be held indefinitely in memory. However, if you use WithDeadline or WithTimeout without calling cancel, the goroutine will only be held until the timer expires.

This is still not best practice though, it's always best to call cancel as soon as you're done with the resource.