ついAccidentally, Unconsciously, Against one's better judgement

Both describe actions that occur without full conscious control, but they differ in nuance. いつの間にか emphasizes that something happened without the speaker noticing, e.g. いつの間にか春になっていた ('Before I knew it, spring had come.'). つい highlights that an action was done inadvertently or against one's better judgment, and often pairs with 〜てしまう or 〜ちゃう, e.g. つい甘いものを食べてしまった ('I ended up eating sweets without meaning to.').

つい and てしまう・ちゃう can both express doing something unintentionally or against one's better judgment. つい appears at the beginning of a sentence or phrase, while てしまう・ちゃう is a verb ending. They are often used together to emphasize the nuance of regret or unintentionally, e.g. つい食べてしまった・つい食べちゃった would imply 'I ended up eating it (even though I didn't mean to).'.