A breaking example might be trying to find lines containing an "=" in a file:

    grep = my_file

Also a problem is the syntax for running a program with environment assignments that apply only to the program:

    env1=foo env2=bar env3= my_program

Note that under POSIX rules, "env3" here is assigned a zero-length string. Making these sorts of assignments work with spaces around the equal signs would open up a can of worms.