Oh no!! That ones got me before too, Yes almost always need grounds connected!!!
Oh man I bet you slapped your forehead when you realised!
Cool though sounds like that problems solved. Its usually pretty safe to connect grounds, its usually important.
If you have two arduinos connected to usb which will talk to each other and then put one of them on batteries they may stop talking. Because the grounds were being shared through your computer via USB
I have only come up agains a couple of times where I couldn't bring grounds together, and they were when needing to maintain isolation between two sides of a DC DC step down.
Actually its a good question.... Can anyone suggest reasons when you could not bring grounds together? Id like to know more about these situations, cause I kinda work on the idea that its almost always ok and if its not the case I should know!