I wouldn't really call this a newb moment but I cant really remember anything that I've done for a long while worse than this. During my most recent playthrough (my expert playthrough) I had lost to Plantera for the 5th time at least but that 5th time was the time I needed to notice that planetera slows down when traveling through blocks. Eureka moments are a bit asinine but satisfying at the same time.
Edit: ...... OMG I COMPLETELY FORGOT ABOUT THE NEWBIEST MOMENT! This is more an inexperience newb moment rather than a random act of stupid. I had an obnoxious time with the yoyo glove when I was looking for it. If I could find the skeleton merchant after damn near a half an hour, he wasn't selling it. So I decided to trap him and Wait. 2 hours later he still wasn't selling it much to my frustration. So I decide to just casually look for him.
Over a period of 2 days and trapping him again for several more hours he was not putting out the yoyo. I looked at the wiki multiple times but there was no indication that I was doing anything wrong........ until I had a Eureka moment and looked at the rarity of the yoyo glove. It was tier 4 which is early hardmode. In the early days of hardmode, niether the wiki or gamepedia actually directly stated that it was a hardmode item so when I saw that I had completely wasted my time for 2 days, I could only look in disbelief. In case it wasn't completely obvious, I hadn't even beaten skeletron at that point. In fact, I was looking for the yoyo glove to beat skeletron because I was having such a hard time against him.
If there's one thing you'll learn about me as you get to know me, it's that I can be very tenacious when it comes to things I want. That tenacity has been the worst of my quirks on numerous occasions. This is just one more of them XD