TL;DR: don't ignore warnings and alarms. You will regret it.

Problems

Missed Errors
Ripple Effect
Fail Fast

Solutions

Enable all warnings
Enable preconditions and assertions in production.
Fail fast
Design by contract

Sample Code

Wrong

array = [];
console.log(array['1'])
//undefined but keep going on

hiddenGlobal = "I am a global"

console.log(hiddenGlobal)
//  I'm a global

Right

array = [];
console.log(array['1'])
//Index Error

noGlobal = "I am not a global"

console.log(noGlobal)
// ReferenceError

var noGlobal = "I am not a global"

console.log(noGlobal)
// I am not a global

Detection

Most languages have warning levels. We should turn most of them ON.

We should run linters to statically analyze our code for potential problems.

Conclusion

If we ignore warnings and code moves on sooner or later it will fail.

If the software fails later it will be very difficult for us to find root cause.

Defect will likely be near first warning and far away from the crash.

If we follow the Broken Windows Theory, we should not tolerate any warnings, so a new issue will not pass unnoticed on a sea of tolerated warnings.