You think you're making a plan. You say 'We hebben een afspraak.' The Dutch person smiles. But later you realize: you just asked them on a date.
Welcome to the minefield of afspraak vs afspraakje. One tiny suffix changes everything.
What's the difference?
Afspraak (without the -je) is a neutral appointment, meeting, or agreement. Business, casual, whatever. Use it for doctor's visits, coffee with a friend, or a work meeting. Safe.

Afspraakje (with the -je) is a date. Romantic. Specifically, a romantic rendezvous. Say 'We hebben een afspraakje' and you're saying 'We have a date.'
So when you tell your colleague 'Ik heb vanavond een afspraakje met mijn vriendin,' they'll assume it's romantic. If you meant a casual hangout, use 'afspraak'.
Real-life example

'Ik heb morgen een afspraak met de tandarts.' (I have an appointment with the dentist tomorrow.)
'Zullen we een afspraakje maken?' (Shall we make a date?)
See? Total difference.
Why this matters

Mixing these up can lead to awkward situations. Imagine telling your boss 'Ik heb een afspraakje met de klant.' Your boss might think you're dating the client. Oops.
The diminutive -je in Dutch often indicates a smaller version, but with afspraak, it signals romance. Just like borrel (drink) vs borreltje (casual drink with friends, but not romantic).
If you want to practice this yourself, the Fluency Tulip is a great place to start.

How to avoid the mistake
- Always think: is this romantic? If yes, use -je. If not, keep it plain.
- When in doubt, say meeting or ontmoeting for neutral situations.
- Listen to how Dutch people say it. They'll never use afspraakje for a dentist visit.
Goed bezig! Stap voor stap wordt het makkelijker.