Recently I have lost 2 reputation points for merely looking at a question, comments and an answer (1 point for reading the question "Large cardinals in $ZF$ + $DC$ + $AD$, and 1 point for reading the question "How many of the true sentences are provable"). Why (please explain)?
To summarise the conversation in the comments, it seems Thomas may have accidentally downvoted some answers while browsing on a phone (I've done the same, especially when the screen reflows because of MathJax rendering), and downvoting an answer (but not a question) subtracts 1 reputation point. Such downvotes can be undone within 5 minutes, and only after this time if the answer has been edited in the meantime.
This is a CW answer - feel free to improve and add as needed. The question is already sufficiently answered - the intention of this answer is to collect sources where one can read more about the topics brought-up here and various related issues.
- How can reputation be gained/lost: How does "Reputation" work? This is a very detailed FAQ post on Meta Stack Exchange. A shorter summary can be found in the help center
- About the reputation tab: How do I read the history of my reputation?
- Downvoting an answer costs 1 reputation point: Why do you lose reputation for downvoting answers?
- Votes are locked - in the sense that one cannot come back later and change their vote, unless the post was edited: Why do votes get locked?
- Can accidental downvote be removed: Reverse an accidental downvote