Of course it can. Software is complex and that complexity can cause all kinds of problems, as can the fact that the networks linking computers are unreliable, but software is fundamentally deterministic. If you write a piece of code that returns a temporary failure when it can't look up whether a user exists, that code will not mysteriously change itself to start returning permanent user does not exist errors. (Now, if your overall stack is designed in such a way that you can't reliably tell the difference between lookup failures and users that don't exist, you have a problem - but the problem is with the design of the system, not some inherent problem with software.)
Note that this is rather different from physical, mechanical systems which can fail in all kinds of exciting and unpredictable ways due to physical wear and tear, things getting jammed in places, component failure, etc.
That's true, but human behavior is also fundamentally deterministic, and those two observations are about equally useful.
> Note that this is rather different from physical, mechanical systems which can fail in all kinds of exciting and unpredictable ways due to physical wear and tear, things getting jammed in places, component failure, etc.
> that code will not mysteriously change itself to start returning permanent user does not exist errors
That is true in a perfect world. In the current world, there are all sorts of ways that code implemented one day does not run the same the next day. Say the code is in an interpreted language and an unrelated sysop updates the language runtime in a way that changes the behavior. Again, in a perfect world that doesn't happen, but that is not always the world we live in. I have great sympathy with people who treat software systems AS IF they were "physical, mechanical systems which can fail in all kinds of exciting and unpredictable ways".
Note that this is rather different from physical, mechanical systems which can fail in all kinds of exciting and unpredictable ways due to physical wear and tear, things getting jammed in places, component failure, etc.