Imitating other code that is known to work [1]. Lots of testing. Fixing the bug when someone runs into it and complains (a viable last-resort for almost anything besides a Solidity contract).

[1]: For example, when working with filesystems, people write code that they saw other people using. The code may or may not work as designed depending on the specific filesystem. see e.g. https://danluu.com/file-consistency/

Users submit bugs and the reply is sometimes "cannot reproduce." :-)

But the more serious projects I've worked on use analytics and have semi-automated ways for users to send you stack traces and logs when they notice a bug.

Also, it's helpful to have a continuous integration setup that automatically runs integration tests on many platforms.

Usually, by crashing or exhibiting bugs and misbehavior.