One of the investment cards has a bonus that says, "Add 1 influence to an occupied Boomtown space." The explanation below states, "May place one Influence Token on top of a space occupied by an opponent in Boomtown. Both you and that opponent will score that end game bonus."
The neither the card nor the rules state that the influence token can not be placed on top of an occupied Boomtown space that you already occupy.
The rules imply that you place the token on a space occupied by another player but does that mean you can't place it on a space occupied by yourself? I know this seems obvious but just curious if you can place on a space occupied by yourself.
Granted, it would have been better if that card said "Add 1 influence to a boomtown space occupied by an opponent" but there's only so much room for text on the cards & tiles.
I think there were a couple of other investment cards & boomtown tiles which have some further details in the rulebook.
I personally haven't found it to be too much of an issue though. I've played the game 8 times and I always go thru all investment cards and boomtown tiles with all players before beginning the game and I use the rulebook as a guide to make sure I cover any details the cards/tiles don't cover (or clarify what they do say). Most are pretty clear though