Your humility is pretty awesome.
As a simple example, paste alert("haha") into an form in your system that will (eventually) be viewable to someone else. Let's say it's something innocuous like the quantity field on an order form or a comment on a comment form.
Now, none of those characters will attack your database through SQL injection, right? So what happens when you (the administrator) views your latest orders? Basically, that javascript is now operating in the security context of you, the administrator, not in the context of the attacker that submitted it. It could do something worse than alert()... for example, it could grab your session token or a cookie's secret value (depending on the cookie) and send it off to , and now the attacker can become you.
Or, it could do the same thing to another user on your forum when they look at the evil comment.
This sort of attack is easy to do and broadly considered XSS (cross-site scripting). There are related areas of attack, like cookie forgery, referral, etc attacks. The OWASP string replacement guidelines (or my safify.js) can help with this, but ultimately string sanitation has to make sense for the context (i.e., browser bad characters are different from SQL injection bad characters).
And, you have to think about the weakest link... us poor humans, to whom similar 𝖻𝗎𝗍 𝗇𝗈𝗍 𝗊𝗎𝗂𝗍𝖾 𝗍𝗁𝖾 𝗌𝖺𝗆𝖾 𝗎𝗇𝗂𝖼𝗈𝖽𝖾 𝗅𝖾𝗍𝗍𝖾𝗋𝗌 𝖼𝖺𝗇 𝖻𝖾 𝗎𝗌𝖾𝖽 𝗍𝗈 𝗍𝗋𝗂𝖼𝗄 𝗌𝗈𝗆𝖾𝗈𝗇𝖾... 𝗆𝖺𝗒𝖻𝖾 𝗍𝗁𝖾𝗒 𝗍𝗁𝗂𝗇𝗄 𝗂𝗍'𝗌 𝖺 𝖽𝗂𝖿𝖿𝖾𝗋𝖾𝗇𝗍 𝖴𝖱𝖫 𝗍𝗁𝖺𝗇 𝗂𝗍 𝗂𝗌, 𝗈𝗋 𝖺 𝖽𝗂𝖿𝖿𝖾𝗋𝖾𝗇𝗍 𝗎𝗌𝖾𝗋𝗇𝖺𝗆𝖾.. (paste that into vim to see the actual characters in the preceding sentence.)
𝗁𝗍𝗍𝗉𝗌://𝗀𝗈𝗈𝗀𝗅𝖾.𝖼𝗈𝗆/?𝗊=𝗇𝗈𝗍+𝗐𝗁𝖺𝗍+𝗒𝗈𝗎+𝗍𝗁𝗂𝗇𝗄
None of those unicode characters will usually trigger any blacklists, but because it "looks" right, sometimes can even trick security-aware hackers. (see also punycode). What if someone spoofs someone's username on github?
There's lots and lots of interesting ways to attack websites. It's tough to keep track of them all.
Another example. Your 404 page..
404
Sorry, /x/y/z doesn't exist.
Now, someone says "Hey, can you please visit this site?"
https://yoursite.com/this-is-a-long-url-thats-hidden-in-a-an...