Are detail "mistakes" real mistakes?

Many times as we reread comics, it's fun for me and others to look at details or things we might have missed reading through the comic the first time. Though when I see things that look like mistakes to me done by professional artists, I wonder how much of a big deal is it to get away with it and hope the reader or editors don't notice. Here's an example from the graphic novel Daytripper. (Which i recommend getting since it's a great story and has great art)

Look at the "Congrats! Jorge" on the birthday card in the first panel and notice that the letters in"Congrats!" is lined up differently with the "Jorge" letters in the 3rd panel. In the first panel the "J" in "Jorge" is right under the "n" and "g" in "Congrats!" In the 3rd panel, the "J" is under the "o" and "n"

Also notice the photograph the birthday card is covering in both panels. In the first panel, most of the upper right is covered up. In the 3rd panel, it's just the tip of the corner covered up.

Do these count as real mistakes to you? Or just let them go since details like this are not the main focus of the story and most readers may not notice? I noticed a similar mistake done in the graphic novel Watchmen, which is considered the best graphic novel ever and yet, it's still highly praised for what it accomplished.

I guess I feel like since these artists are "professionals" they shouldn't do things like this? Do you consider it acceptable to make a mistake when showing something the first time like a sign, signature or other little details and it doesn't show up exactly the same when shown again in another panel without explanation?