1. Socks in Sandals
No one would dream of wearing socks and sandals at home, so why do it when you're in Spain? It’s the surest sign that you're "not from round these parts". Other fashion no-no's include baseball caps when it isn't sunny and t-shirts that announce the city you come from.
2. Speaking unnecessarily loudly and refusing to attempt to speak Spanish.
It might seem hypocritical for Spaniards to frown on tourists for talking loudly when they speak at a high volume themselves, but remember that you're speaking in a foreign language - and this stands out more. No one expects all foreigners to speak fluent Spanish, but flying up to a Spaniard and barking "Where is the train station?" without at least asking first (in Spanish) if they actually speak English is plain rude. Oh, and no, speaking louder does not make you easier to understand!
3. Eating and going out at un-Spanish times.
The Spanish eat late and go out even later. If you don't follow suit you will be (a) eating alone and (b) the subject of ridicule.
4. Reading a map and travel guide on street corners.
Do you want to advertise the fact that you are unfamiliar with the city and are perfect prey for pickpockets and muggers? No? Then don't carry your map and travel guide for everyone to see. Stop in a café to check where you're going.
5. Skin a shade of pink in March.
If you're in Spain for two weeks, what's the rush to get a tan? Spending all day on the beach on your first day without any sunscreen will leave you burnt to a crisp and in so much pain that you don't dare sit on the beach again. The burn quickly fades and by the time you are home you're as white as when you started.