About only 1 police with arrest:
Kleene was originally Chiba, but Yuriko disguised as her role which left only xpon with arrest.
To GMs (specially Mystery since she is the next one): READ-THE-RULES.
I'm not gonna go into specifics with Kain's work (I'll leave you an expression which probably sumarises it well -->

) but she made a lot of mistakes. So, read the rules... read again the abilities of the roles in the game if possible too, don't assume you know all of them cause you may miss something. A GM doesn't have to have a super memory to memorize all the rules or be a genius to process all what's happening in the game, just by being responsible, organizated and commited is enough to handle GMing.
Don't GM just for the sake of GMing or because everyone says it is fun (Which is true), commit to the players, cause they expect you to do your best, be organized with all the info, because there's a lot of info and actions that are sent (or not) every phase and you have to process ALL of it and be responsible, with the time you set phase change (Most of the players change their life schedule according to it, and this
IS TRUE). If you can't make it on time on one phase, and you advised earlier about it, everyone will understand (Round 25 comes to mind as example on this). If you start postponing it constantly, players will not like it much. So select phase change wisely, and change it if it is bringing difficulties, it is much better than posting late.
CTU's, Cheesus's and this one from Kain are clear examples of this: Constant mistakes made, unreplied PMs asking important info about the game or asking your results from 3 phases ago (Yes, this has happened), GMs not being able to finish their Rounds...
Also, when it doubt, ask. Don't be afraid to ask some veteran (or newbie, why not?) player about something of the game because you're GM and "supposed to know everything". It is better that you ask and at least get a second opinion about it, it will surely help in some way. More examples: On Round 3, Akonyl (GM) asked me (I didn't play that Round) about my opinion on a Sonoko/BO matter. On Round 6, I (GM) asked Akonyl (Suicided on Night 1) his opinion on a healing/protection matter.
That's all. You might also take this as some kind of ranting over GMing but I know I'm not the only player who hasn't liked some of the last GMs and I want this to change sooner than later...