Would it be a problem at the myanmar border (and in the future when i will apply for a new double/triple entry visa) if i have overstayed my first entry by one day?

It will not be a problem, but they will stamp your passport with overstay and fine you 500 baht. But that is all.

Of course the overstay stamp might be a problem entering other countries, but that depends on your nationality and the country you want to enter. For a Western European it will not be a big problem to enter the US with such stamp, while for example an African might have a bigger problem entering the US or another Western country.

No problem at all - my current passport is littered with overstay stamps - and the fact that I've paid my fine. I've never had an Immigration Officer comment on them when I'm at a border or when I'm in their office getting a Visa extension.