There has been a lot of talk about scumslips. In almost every mafia game, at some point a player is going to point out a post of another player as "scumslip". Before we go into details concerning the merit of these arguments, we first need to get our definitions straight.
So what is a "scumslip", and how can we differentiate it from the more general "scumtell"? Scumtell can describe pretty much anything that makes you think a person is scum. Interactions with other players, voting history, activity level, defensiveness etc. can all be scumtells. "Scumslip" is used in a much more narrow way. There are basically two kinds of behaviour that are usually classified as "scumslip":
1) Accidentally writing a scum-related thing in your post instead of a town-related thing. Captain_Frisk's "0% chance" fits here, as well as the <b>scumslip</b>.
2) Making an argument based on information that one should not have access to. Examples can be knowing the exact size of the mafia team, knowing that there is a SK, having information about night actions and so on.
Now I don't think we even have to argue about 1. If you're an active mafia player, just ask yourself: how many times have you accidentally written "scum" instead of "town"? Does it happen more often if you are indeed scum? Advotcates of scumslips might want to point to the original Captain_Frisk scumslip, saying "but there it worked"! Well, sure. Every game features around 25% mafia, so if you're using a scumslip argument often enough, it will hit scum at some point. The same is true though about the argument "that player used T as the first letter of their first post". If you are lynching based on this, you will be successful sometimes, but that doesn't say anything about the validity of your argument.
The second kind of scumslip seems more legit, at least at first glance. If people are using information that a standard town member couldn't have, it has to come from somewhere, right? How this argument is ultimately flawed tough can be seen when it's taken to an extreme: You could use this argument to lynch a claimed Cop with a guilty result. Because, obviously, the Cop has information that mafia would have - namely, who one mafia player is - so they have to be scum!
This leads us to one of the reasons the "scumslip" argument is wrong. There are mulitple ways in which you may get to know more about the setup than others do. You may be a town PR. You may have made some connections from things that happened earlier that others haven't made. You may have read the setup post carefully.
Secondly, there's the kind of posts that are based on knowledge that isn't actually there. You might appear to "know" something others don't, but only because you didn't think everything through. Another thing happens when some people believe in a theory, and they trust this theory so strongly that they assume it to be true. This often happens when players act as though they know there's a SK, even though they don't.
Thirdly, the argument is flawed because scum is careful. They will think twice before posting something. That means that they are actually more likely to eliminate obvious errors in their reasoning than town players are. By saying "this is a scumslip", you have to assume that scum let this slip happen, and more often than not, they won't. There are cases in which they actually slip, but by the points before, these are mostly indistinguishable from instances where town just makes a bad argument.
So ultimately no, scumslips don't exist.