In a chat, Lisa asks John if he's seen the new problem in their computer program.
John's response could mean five different things depending on what tense he puts on the verb fix:
lisa: Have you seen the new bug in our program?
john: Fixing it (now)
john: Have fixed it (some time
john: Fixed it (can even tell
you when I did it).
john: Will fix it (soon, later,
john: Fix it (yourself)
If the tenses are wrong, the one you talk or write to starts guessing what you really meant.
...and that's when the fun starts :-)