What to do if you overstay your Myanmar tourist visa
If you overstay your Burmese tourist visa, it's not that big of a deal (at least as of early 2019), you simply have to pay some fines. However, don't plan on doing this as it can complicate travels within the country
Here's what happens:
Upon leaving the country (either airport or land border crossing), you will be asked to pay a fine of $3 USD per day for each day overstayed (and $5 per day after 90 days of over overstay). There is an office at the Yangon airport specifically to handle this, and after you've paid you'll get a stamp in your passport indicating that you're "cleared" to leave. Payment is by cash USD only
If your visa expires while you're inside the country, it can complicate your travels a little bit because you won't be able to fly within the country as domestic airlines will check your visa when you check in. Buses and trains are supposed to check your visa as well, but they're much more lax (especially buses as they're all privately operated and they want money). Also, hotels and guest houses are not supposed to let you stay if you don't have a valid visa, but again most smaller guest houses will let it slide
Some things to note:
The tourist visa can't be extended, so overstay is the only way to visit Myanmar for longer than the visa allows
Try not to overstay for more than 15-30 days. Within this time it's a small administrative issue, but the longer you overstay the higher the risk of law enforcement making a bigger deal out of it
You're not going to be put on some kind of list if you overstay. It happens regularly to many tourists and as far as I know no one gets banned from entering Myanmar just by overstaying the visa