It can be serious or not, just things that make you mad or annoyed every time you look back on it because you know you could have 100% prevented it XD. Or just things you could have appreciated more.
For example, I once owned a lego Mars Mission toy set with my two brothers. Between us, we had almost completed the entire set. Then as time went on, the pieces got mixed together, along with some pieces from a Star Wars set (idk). We tried putting them back together, which got them even more mixed up. Then we just stopped caring, and eventually the whole thing was lost (thrown away probably). Now, not only is that particular brand of lego no longer sold in any store, the only replicas are on Amazon as antiques. I bought my pieces at Walmart for about $15 total, and now they're worth high in the $60s. One of my brothers' pieces, which was around $65, is now worth at least $400. So yeah, every time I think about it, I wanna kick something.