I replicated this issue in the demo wordpress account. If one creates a paid event with a financial type that includes taxes, enters a price (without using price sets) and books a person, then opening the contribution and clicking the 'save' button behaves correctly.
The problem appears when the event price is a price set. Each time one opens the corresponding contribution and clicks 'save', a new accounting line is created with negative the amount of taxes.
Example after opening and saving several times:
(the total amount should be 400 SEK, but now it say 14 SEK)
This happens when the price set is used with an event, but also (at times) when directly creating a contribution with the same price set as above, but without registering to an event.