If somebody really wanted to cause true mischief and was an expert in an aspect of, in my case, biodiversity, it would be possible to cause mischief, but the fact of the matter is that I have never encountered somebody with such a level of expertise who actually wanted to cause harm.
The other thing is that, if there's an outlier observation that somebody very cleverly slipped into the system, other experts would be excited about this. Then they would go and check and fail to find that this could be supported.
The system has a lot of checks and balances. I wouldn't say we never have to worry about it in the broad realm of citizen science—