The IRS has always responded in a really positive way, so it is really worth talking to them (well, unless you think you may be in real legal trouble... I guess "don't talk to the cops" applies here too...) if you're in the situation where you think "oh, yeah I messed up" or "oh, yeah they messed up" it's probably best to just reply... but yeah if you think it is serious talk to a tax attorney... (not an accountant, which isn't a tax attorney).
The "they messed up" for me was a crypto thing, where I actually took a small net loss, and didn't file (because you don't need to, for capital losses, from my understanding, though I guess you should) and they thought I took some insane gain b/c of the $ value of the trades.
I just mailed them the trades, said "I didn't submit the form for this b/c I didn't think I needed to for a loss" and they said "yay, thanks, see ya" a bit later. Positive experience.
Agree, most countries (including India) simply opens their portals, you sign in, check all numbers, report anything missing like income or deduction, and press the final Enter.