[nobody314159]

They don't even have to change anything - a VISA isn't a guarranteed right of entry.

You just need minimum wage guy at the airport desk to not like your face and you don't get in - no matter how many VISAs you have in your passport. You can of course try and correct the problem from the other country after you have been denied entry - if you can get an airline to carry you.

[hugh3]

I've never heard of anyone being denied entry, in practice, because the minimum-wage guy (actually immigration officers earn a median salary of $46,0000) at the airport doesn't like their face.

It's true that they legally can deny entry to someone with a valid visa, but in practice it has to escalate a few levels up the chain of command before they can do that. A low-level flunky trying to deny someone entry for specious reasons would get into trouble.