Back at home in Ontario, Canada, I'm fairly certain which plants I'd consider weeds and therefore undesirables in either the flower or veggie garden. Here in the subtropics I'm not so sure at all. I can spot the (non-ornamental) grass and try to remove it from the flower beds, but there are other plants that look like something you might actually buy and grow.
I've removed some 'viney' things that appear to be crawling over legitimate garden inhabitants, but last week there was a larger one that I wasn't sure about, but which I left. Today I walked around again and now I'm glad I left it alone - it's flowering nicely. I googled it and I discovered that it's actually a bougainvillea! I also discovered that its sap can cause skin rashes similar to what poison ivy and poison oak cause, and I'm familiar with those. Glad I made this discovery the easy way by reading about it and not by experiencing it !
(60mm f16 1/30 sec ISO800)