I think of loop as being the arduino's substutitue for main in C. So I declared some variables at the top of loop. Bad idea, they get re-initialize each time around the loop. Instead declare as globals before any subroutine. Not a big deal, but took a few minitues to fugure out what was happening.

For me it's the other way around: I wish I knew about Arduino's and AVR's before they tried to teach me electronics in secondary school. Learning Ohms law without ever having seen or used an actual resistor was not a pretty exercise. Back then it would be ideal to have an Arduino around to play around with.

Because of this, I would have like to have known more about basic electronics. Simple questions as: why should I care about current? (because things blow up if you don't) Why should I care about resistors? (same here and other stuff) Why should I care about caps? (all kinds of reasons)

Having the ability to play with micros (and frying them in the process) has tought me most of these things but I wish I had learned them sooner. (Classic chicken and egg problem I guess)

I wish I'd known how much FUN it was :) If I had I would have started playing with this stuff much sooner!

I wish I had known that every math class I would ever take I would use again, and more than once, and I sure as hell wish I had paid a bit more attention.

I wish I had a lot of hands on experience programming and using FPGAs. They are the new wave of embedded system development and I have only basic class experience with them.

I wish I had been taught how to use all of the modules of Microcontrollers in classes before my boss expected me to use them all.

I sure as hell wish that my university's electrical engineering department started design classes before senior year.

I wish i had arduinos and "make things talk" when i was a kid. I would have automated everything!

I have used the Arduino a fair bit and I wish I had known how difficult serial IO was on arbitrary pins. I eventually settled on a 3rd party library but it took weeks to get to that point and even then it was not rock-solid reliable.

I'm with Kortuk. I wish we'd started simple design classes before final year of university, but specifically I wish we'd gotten to use an Arduino board in my classes!

We used a specially configured board designed at University of Wollongong based on an old Motorola chip, all programmed in assembler via a monitoring application. Pain in the arse!

Of course, I look up the course content for digital design now and they're all using Atmel AVR chips. sigh

I also wish I had more time to tinker with 'em, 'cause that's the best way to learn!

I wished I'd had a clearer understanding of Fuse bits. I spent half my time early on terrified that I'd brick my mega32. Oh and ditto with regards to serial IO.

The CAN bus communication scheme and CANOpen.