No shit, there I was (see Libby, I do listen to you), standing in a field, giving serious consideration to an upright concrete pipe. It was old enough to qualify as a historic resource (marking on it indicated that it was erected in the early 1950s), but I am pretty sure that the authors of the National Historic Preservation Act weren't thinking of concrete pipes when they were writing the legislation. Still, it met the criteria necessary for federal agencies to have to consider impacts to it. Nonetheless, we were in the construction phase of this project, and this particular historic bit of field hardware hadn't been included in the considerations regarding historic impacts because the archaeologist leading this segment of the survey had not taken note of it.
Now, if you were to see this pipe, it would likely escape your notice. It's a vent for an irrigation system, the sort that is ubiquitous throughout California's agricultural lands. It's the sort of thing that just forms part of the background of most of California (remember, despite it's reputation as an urban beach-covered state, the vast majority of California's land is landlocked and rural). And I think that this might be the reason why the thing was overlooked by the people who should have at least made note of it.
Archaeologists are similar to medical doctors: we all get a basic course of training in anthropology* and archaeological methods, but then we gain a specialty. We first divide into historic and prehistoric archaeologists: the historic archaeologists study people who left behind written records (so, in California, they focus on the region after the missions began to be established in the late 18th century) and prehistoric archaeologists focus on people who had no writing system and therefore no written history (so, the Native Californians before the Spanish showed up)**.
And that is where the problem likely came from in this case. Prehistoric archaeologists tend to be very cognisant of the likelihood and possibility of prehistoric artifacts in a given area. They can identify historic materials, sometimes they are very knowledgeable about them, but they tend not to notice the historic materials that are hidden by the landscape. So, a historic irrigation feature in an agricultural area may be missed by a prehistoric archaeologist because of the fact that there is nothing odd about it, and it is precisely what one would expect to find in the area.
To be fair this goes both ways. I have met historic archaeologists who have missed carved rock art that any historic archaeologist would immediately pick up on, as such rock art sometimes looks like it might be a weird erosion pattern. In one particularly alarming case, I met a historic archaeologist who didn't comprehend how to identify a bedrock mortar*** - one of the most common types of prehistoric archaeological features. So, this can go both ways.
Still, when you are doing fieldwork, it is good to be aware of one's limitations, and to be extra-vigilant based on them.
*With the exception of many Classical Archaeologists, who forgo training in anthropology and focus instead on history.
**It gets more complicated, though. Most North American prehistoric archaeologists can identify prehistoric sites and materials across North America, and most North American historic archaeologists can do likewise for their time period. But, nonetheless, there are regional divisions. I am focused on California, for example, so while I can work competently in other regions, I will have more to learn and will have to work harder to do a good job than I would in California. And this is without even getting into material specialties - some people are specialized in stone tool analysis, others in faunal bone identification, others in geomorphology, etc. etc. etc.
***This was made especially annoying by the fact that he insisted on sending me to the field with "a more experienced archaeologist" who actually had less experience than me, kept missing bedrock mortars as well as other types of sites, showed up severely hung over and couldn't even manage to get to the correct location for work. What's more, if the guy who had sent us out had bothered to listen to anything that I had ever said to him, he'd have known that I am a prehistoric archaeologist with many years of Californian field experience. In other words, he'd have known that I am perfectly capable of identifying a bedrock mortar.