The ONLY problem I have EVER had loading at the upper end ( MAX LOAD) was when using a stiff load in my 25-06, RE22 powder and a 100gr BT. I worked this load up during early spring, it was fine at those temps, in summer though, that load blew primers and locked the bolt solid.
From that experience, I work up all my loads in summer, if the temp is lower than 30 C I don't bother doing work ups.
As far as dirt, water or oil goes, raising pressure enough to cause an issue is very rare.
I have oiled, greased and wet cartridges to SEE what happens. The ONLY visual thing that occurs is the EXTRA bolt thrust, it shows up as an ejector mark, but the primer pocket DOESN'T enlarge any. Even over the chrony there is no significant increase in ACTUAL velocity, with the Pressure Trace, there is no large pressure variation either.
I have had an issue with 'stuff' left in chambers prior to shooting, patch threads, bits of brass, bullet jacket and even a plastic tip from a bullet. They all left impressions on the brass cases, no other issues were noted, however.
As long as you stick with published data, 99% of the time a max load will/should be perfectly safe, it the 1% you need to look out for, such as my example above regarding temperature when shooting.
Hope this helps.
Cheers.