s not only occurs, it's one of the more frequent triggers of lucid dreaming
(rare enough, granted). Y'see, as is noted above, the subconscious
spontaneously produces word
s to fill whatever text
-bearing objects may appear in your dream every time they come into the range of your dream-sensory-organs - sometimes the text is rambling, surreal
; sometimes it's like a program loading variables from a part of memory containing unwiped garbage from previously-run programs; and sometimes it actually seems to make sense.
It is in the first and last instances where the possibility for the trigger emerges; as the illusion of meaning and comprehensibility occurs, you are presented with strings and phrases which can be read and remembered. Often this recollection doesn't go so far as the capability of making notes of what the words were after waking, but it tends to be persistant enough for you to realize, after looking away and back at the text-bearing object, that the words you are now reading are not the same words which were just a second ago on the page / storefront / street sign / whatever. And you do a double-take; and they're changed again.
At this point one of those "questioning reality" moments occurs and you either
a) dismiss your dream-perception as faulty, concocting some rationale for the words having changed,
b) realize that you're dreaming, and immediately wake up, or - if you're lucky...
c) realize that you're dreaming, and make the best of your situation.
Keep reading everything you see in real life, and the habit may persist in your dream-life, leading to increased opportunities of that mythic commodity - lucid dreaming.