Mission-critical systems use SRE principles so no one is debugging code at 4am. If you can't solve a production issue without looking at the source code, you'veong since failed at building a mission critical-system.

Even if your app is 100% bug free and fault-tolerant, sometimes an external system feeds it invalid data and you need to figure out how to contain and revert the damage. That was my last on-call involving reading a lot of code under time pressure (thankfully, not alone and not at 4 am).

All commenters who said "It's your own fault for writing crappy code, duh!" perhaps never worked with anything sufficiently complex and expensive.