The bug

Command blocks trim spaces when checking syntax, but do not when running commands (which is hopefully intended). This leads to situations where no syntax error is shown, but the command will not be executed.

Reproduction steps

1. Paste the following in a command block (include the trailing spaces)

   setblock ~ ~1 ~ stone replace    
   

   No syntax error is shown

2. Activate the command block
   It fails due to the trailing spaces