This is a feature to help when you have a small typo, but as you noticed this sometimes results in actual wrong answers getting accepted. There are two scripts that will help with this (don’t use them at the same time).
This script will cause the input box to shake if your answer is “close enough” instead of accepting it.
[Userscript]: Double-Check (Version 2.x)
This script allows you to mark your answers right or wrong for situations like this. It also has the “close but no cigar” script built in as an optional setting.