The tiles between the trees right in front of Littleroot meaning you can't get in for starters. I'm technically able to enter Littleroot but I am stopped one step in.
Mauville and route 117 have the vast majority of them, I don't know how to post a screenshot but for Mauville the southern section of the main road cant be walked on, the left side of the gym next to the trees has two such tiles. As for the right side, the grass next to the pokemon center is completely booked off as well as the row that has the little boy by Rydel cycles who says Rydel went to the orange islands. The blockage there is the entire row of the town except for the road.
Actually scratch all of that for Mauvile I think all of the tiles in Mauville might be like that with the exception of the grass in from of the flowers at the entrance to Mauville, the sole grass tile beside the Game Corner building, and the row of grass tiles on the northern side of the road. The same row as the Mauville town side is on.
ALSO very important to note is that you can pass through them left/right but not up/down.
As for Route 117, I'm going to break into three sections for my own sanity. The three sections will be daycare/twin lakes/Flower garden I am going east to west.
In the first section with the daycare, all of the southern grass is like this, also the daycare is like this too meaning if you step on the door you have to restart the game. the part with the lady and the berry patch you can't walk past the lady.
Of the lake section pretty much all of the grass is like this.
For the last part the flower garden section. the flower gardens cant be entered and a lot of the passage to Verdnturf is like this
I have yet to encounter any tiles like this besides the aforementioned Littleroot entrance.
Edit: corrected spelling/grammar.