Fees that are usually not included in most calculation (but should be!): evictions (expensive!!!), lawsuits, general legal fees, asset protection (business entity structuring), insurance, large capital outlays (e.g. new roof), etc. Something to remember is that expenses could be 500% of gross rents for certain months. The 50% number is a conservative average.
Not to speak for him, but when MikeOH uses the 50% number, he is basing this on the long term over many properties. It is the expected cost of operation and does not necessarily hold true for one property for one month. In fact, in his book, he suggests (and I agree) that one is most vulnerable with few properties, because cash flow from other properties cannot serve to soften the blow of large expenses on one property (again, going back to the idea of an average cost).
So, in other words, in infinite time (or over infinite properties) he is estimating operating expenses will cost around 50% of gross rents. The shorter the time or the fewer the properties, the more variance that is possible. This might mean you have expenses at 1% of gross rents or 100% of gross rents. But the average (given unlimited time) would end up being 50%.
I believe Mike is accounting for some of this possibility for variance by using a slightly more conservative number than most. This is definitely a good idea that you should replicate.