Pretty much explained in the title. If you manage to have enough CDR that you can activate E before the 5 second timer on the previous E finishes, you won't get the bonus damage or the auto reset. 100% reproducible.
RiotMANWOLFAXEBOSS5 votesThanks for the report. It's fixed in the next patch :)
Slamurai Jack1 votesYou guys move fast.
Yxion the Yordle1 votes
Good to know, might avoid playing Mundo for the time being. Will have to check it out in custom match.
Slamurai Jack1 votesIt doesn't really make much of a difference. You need to have pretty high CDR to even get the E CD low enough to do it, and you can always just wait out the remaining .5 seconds or so of E and then use it. It has a very minimal impact on gameplay since most Mundo's will never have enough CDR to get E below a 5 second CD anyway and the bug has a very easy workaround. I'd be honestly surprised if anyone in testing or on the PBE even noticed since you need 30%+ CDR to even find the bug.
Yxion the Yordle1 votes[{quoted}](name=Slamurai Jack,realm=NA,application-id=LqLKtMpN,discussion-id=76nTuvhw,comment-id=00000000,timestamp=2015-10-19T15:16:28.593+0000)
It doesn't really make much of a difference. You need to have pretty high CDR to even get the E CD low enough to do it, and you can always just wait out the remaining .5 seconds or so of E and then use it. It has a very minimal impact on gameplay since most Mundo's will never have enough CDR to get E below a 5 second CD anyway and the bug has a very easy workaround. I'd be honestly surprised if anyone in testing or on the PBE even noticed since you need 30%+ CDR to even find the bug.
I like having max cdr on 
:c
Slamurai Jack1 votesAs do I. But the League community as a whole doesn't seem to value CDR very high on Mundo for whatever reason. Their loss.
FFiresoul1 votesCDR is kind of nice, but hard to achieve on Mundo. The way to do it would be having both locket and ohmwrecker in your final build, but it's hard to justify prioritizing these over boots/cinderhulk/spirit visage / randuins / sunfire cape /...or even arguably a GA or dead man's plate or warmogs. There are just so many other items that Mundo likes more.
Slamurai Jack1 votesThat's what runes are for friend. Fuck all those other items. CDR/lvl glyphs and quints.
EndlessSorcerer1 votesI like Health Regen Quints and Scaling CDR Glyphs personally. Those plus Spirit Visage and masteries bring you to 30% CDR, which is enough for me.
Slamurai Jack1 votesThat's also a solid option, although that means you can't get the full defensive tree since you have 4 points in offense. I find 1-29 masteries works really well and I just go cdr/lvl glyphs, 2 quints, and then 1 defensive quint based on the matchup, typically armor. That leaves me with 35% CDR which feels sufficient while still giving enough early game power as to not be vulnerable and the defensive masteries scale really hard as the game goes on.
EndlessSorcerer1 votesYou can skip the 5% CDR in masteries without much issue, leaving you at 25%.
I used to use 5/25/0 masteries before they made the alterations to the defensive mastery tree and I haven't really played Mundo since those changes (I've just had other people I wanted to play).
Honestly, now that I think about it I would be tempted to use 0/30/0 masteries on Mundo again.