Mistakes happen, and a common one is the substitution of table salt for kosher salt.
First, there is nothing really "kosher" about kosher salt. The salt gets its name because it's used in the koshering process, and in terms of meats it's used to pull the blood out of meat to make them kosher. Now, I'm not Jewish so I'm totally ignorant of kosher this and that. But, that's what I know. [Also, kosher salt tends not to have iodide in it too.]
As this picture shows, however, there is a definite size difference between a kosher salt flake and your typical table salt crystal. The kosher flake is that dandruff looking thing on the left. If you can't see the salt crystal, it's near the pen tip.
Since the kosher salt is bigger and "flakier" you get less actual salt per volumetric measure than you do with table salt; think of how a cup of lead weighs more than a cup of water. Thus, if you just use regular table salt in place of kosher salt, get ready for a dose of the Pacific Ocean in your mouth.
To substitute table salt (or vice versa, kosher), use 1/3 to 1/4 less table salt than kosher. The box of kosher salt will also tell you how much less to use and this can vary by brand so pay attention. In general, I use 1/3 less and if things aren't salty enough, I just add more salt at the end.