Slip through the net
Definition 1 of 1
Pro Tip 1/3Metaphorical Net
Imagine a safety net that fails to catch something. 'Slip through the net' is similar, meaning someone or something was missed. 
Despite the safety protocols, a few hazards slipped through the net.
Pro Tip 2/3Negative Context
The phrase often implies a system's failure or an oversight that should have been addressed. 
A few critical warnings slipped through the net, resulting in unforeseen difficulties.
Pro Tip 3/3Not Blame
Using 'slip through the net' doesn't usually assign blame; it points out something that was unintentionally missed. 
I can't believe that error slipped through the net in our final report!