Proverbs 28:18 It will happen suddenly

¹⁸ Whoever walks uprightly shall be saved: but he that is perverse in his ways shall fall at once.

I was checking out some commentaries on this verse, and I read that the phrase "at once" means "all at once, or "all at one time."

Numbers 13:30 And Caleb stilled the people before Moses, and said, Let us go up at once, and possess it; for we are well able to overcome it.
Caleb was saying to the people, "let's go all at one time." 

Deuteronomy 7:22 And the LORD thy God will put out those nations before thee by little and little: thou may not consume them at once, lest the beasts of the field increase upon thee.
The counsel here was that nation-building would occur gradually, not all at once.

1 Corinthians 15:6 After that, he was seen of above five hundred brethren at once; of whom the greater part remain unto this present, but some are fallen asleep. 
A group of over five hundred souls saw the risen Christ "all at one time."

We've seen so many times that Solomon (and his dad, David) counseled his hearers to walk uprightly -- to uphold righteousness -- to be wise and true, and then they would be protected by God:

Psalms 84:11 For the LORD God is a sun and shield: the LORD will give grace and glory: no good thing will he withhold from them that walk uprightly.
 
But what is perverse? Well, it can mean crooked:

Deuteronomy 32:5 They have corrupted themselves, their spot is not the spot of his children: they are a perverse and crooked generation.

And it can mean incomplete, and imperfect: 

Job 9:20 If I justify myself, mine own mouth shall condemn me: if I say, I am perfect, it shall also prove me perverse.

A perverse, or imperfect, unrighteous person can have their life unravel like a ball of yarn between two kittens, when their behavior is revealed to public scrutiny. It takes years and years of hard work, step by step, to build a reputation --- but losing it can happen so very quickly. It can happen "all at once" and suddenly.
If we've been hiding things in our lives instead of walking uprightly, we can expect the "fall" to happen suddenly. What's a Christian to do?
Walk uprightly -- walk each day with Christ. Stay in His word, and talk to Him in prayer. When you can, keep fellowship with other believers.  God will keep and guard you.

Psalm 91:1-3   He who dwells in the shelter of the Most High will abide in the shadow of the Almighty.  I will say to the LORD, “My refuge and my fortress, my God, in whom I trust.”  For he will deliver you ...