The classic errors — and their smell

Some errors repeat so often they have names. Learn their smell and the fix is usually one line.

#The four classic bugs

Match your error to the smell, apply the reading order from the last lesson, and the fix is usually one line.

#The nil-index family

thing.Name where thing is nil is the most common beginner error. The cause is almost always one of three:

1. Misspelled name. workspace.Coinz when the part is Coin.
2. Doesn't exist yet. The object appears a moment after the script runs → WaitForChild.
3. Never assigned. The variable was set in another script or another PlayerAdded.

The error names the exact line. Go read that line, name the nil thing, and pick the cause from the list.

#Syntax slips — type the fix

The fastest bugs are the ones you typed without noticing: a missing local, a single = where you meant ==, a string where you meant a number. Luau is forgiving about layout, but strict about the words.

Local=coins "0"

#String arithmetic and wrong-side changes

attempt to perform arithmetic on a string means a number landed where a string sat — usually player input straight from a TextBox. Fix: tonumber().

"Event fires but nothing changes" is trickier — the code ran, but on the wrong object or the wrong side. A server print shows the server's value; a client print shows the client's. When they disagree, you changed the wrong copy.

local typed = "10"
-- convert before math

#Common mistakes

• Fixing the message, not the cause. Match the smell first — the same error can have three different causes.
• Assuming Studio and live behave the same. DataStores, latency, and timing are the usual suspects. Test live.
• Ignoring which side. A client error and a server error are different worlds.
• Fixing one error, missing the next. Errors often come in waves — fix the first, Play, and the next may surface.

Next: the map of everything you've learned — and what comes after.