Going "behind" something is not immediately a completely binary (all-on or all-off) situation. In other words, in some cases the signal will degrade progressively somewhat before it is completely cut off - if you're paying attention and you realize that's happening, you may have a chance to save things. It's going to depend on multiple variables, eg how big the object is that you're going behind, how close it is, how "solid" it is, etc.
As a rather extreme (and completely stupid) example, last spring I was in a remote location, flying my drone from a beach on a tropical island. This island had a huge, dramatic, rock mountain that jutted up a few hundred yards from me -- picture "the Rock of Gibraltar." It wasn't quite that big, but it was very large, solid stone, steep-sided, high, and near me. I managed to fly my drone around it, and got about half-way around it, I was admirinig the view of the other side of the island which I could not see, was scouting for a nice beach...then to my surprise, the signal started becoming intermittent...the video started cutting out, I began getting "signal loss" messages...
That's when I stopped and had an epiphany: Gee, I guess the signal between the RC and the drone goes in a straight line, and it can be blocked....so me expecting the signal to penetrate through a solid rock mountain may not be very smart and hey, what does the drone do if it loses the signal? If it heads straight back to the home point, it's gonna fly straight into the mountainside...uh-oh.
After quickly dope-slapping myself and yelling "D'oh!!" a la Homer Simpson, I tried to reverse course and fly the drone back around the mountain the way it came, and I was able to. The video feed sputtered back, the signal-loss messages stopped, I got lucky (way more lucky than I realized at the time).
Let's just say this was before I understood how important it was to maintain VLS (I learned a lot of other lessons on that trip...). But even though the drone was pretty much "behind" a stony mountain, I did not completely lose the signal all at once (although I bet I was about to!), it was more of a progressive loss-of-signal that eventually become impossible to ignore (even to a complete idiot). Fortunately, I came to my senses quick enough to get the thing back around that mountain enough to re-establish a decent signal and was able to fly it back home for a safe landing.
Going around a nearby water tower (or cell tower)? I would not worry too much about that. A gigantic, stony mountain? No, that's not going to really work well.