Forgetting what gear you're in is like forgetting to count the shots when using your six-shooter. You look pretty foolish when the hammer drops and the gun just goes "click"...
That said, I do it more often than I'd like, too
I had a Suzuki 550 once that told you what gear you were in. It was an early 80s bike, back when they were putting all kinds of sorcery on the instrument panel.
The next bike I had didn't have that feature, and worse had only five speeds as opposed to six, so I was forever trying to shift into "sixth" until I got used to noticing where the tach was at different speeds in different gears. That's easier to do "at a glance" on a bike with an analog speedometer, because you can just spot the needles and know right off. The digital speedo on the Speed Four makes it slightly more awkward, but I feel less silly checking than I do trying to grab "seventh" gear.
On the Speed Four (mine has stock gearing), I have noticed that if I am in fifth gear at highway speeds, the thousands digit on the tach needle is typically the same as the tens digit on the speedometer.
For example, if I'm going 75 and the tach is around 75(00 rpm), I know I have one more upshift.
If I'm going 75 and the tach is down around 65(00 rpm), then I know I'm in top.
If I'm speeding or otherwise up to no good, then as long as the bike is in a gear that feels right, it doesn't really matter to me.