i was just about to buy this game when i noticed the sale price went up by 40%... what do i do? why did it heppen? why didn't gog tell anyone about it before?

I saw it too, wanted to buy it for 50% off later today. But I'm not going to buy it for nearly the full price.
I hope it was a mistake and they will restore the 50% discount...

There is no way that it was a mistake since it required a deliberate action on the developer's part. IMO there are two main possibilities:

1. someone has made a mistake when inputing the discount amount.
2. this was done to get some quick money from impulse buyers looking through the deals on the first day of the sale seeing as this was the first time A Hat in Time had a substantial discount and many reviews complained that the game didn't have enough content to justify the full price which meant there was a lot of people waiting for a discount like this.

When sales like this happen I usually take my time to think if the deal I'm presented with is good enough which is why I was caught by surprise with this price change. I hope this is an isolated incident since I would hate for responsible consumers to be punished for not impulse buying games. This reminds of when Steam did flash sales during major sale events when a few games where available for outrageously low prices.

This was not a mistake, it was a limited-time 50% sale. I've seen it happen with other games. I think this 50% cut happened because the Steam version is more popular and the game's sales has been slow here. Sometimes GOG will offer the price again towards the end or the last day of sale.

gog.com/forum/general/the_winter_sale_begins_grim_fandango_goes_free_c477e/post257
(Seems like I can't post any links...)

So it really was a mistake. :(

I guess I was wrong on that. I'll eat my Hat now. :-8

Then I can call myself lucky. I had the game on my wishlist and when I saw it was available for 14€, I bought the game. I wonder how many people bought it.