After using several trades with a couple villagers, trades I use frequently have increased in cost and other trades have decreased. However, the GUI does not change to show these changes in costs, but instead stays the same and gets a little buggy when trying to trade.
For example:
Paper is 24 per emerald, but in reality it is upwards of 30 but still says 24. When attempting to use less than 30, it will keep reverting the trade until it says no more trades are available but reopening the GUI will undo that.
An enchanted book is 9 emeralds and 1 book, but when trading it takes only 6 emeralds and 1 book and returns 3.