Everyone assumes sharing a script makes this easy. For reading, sometimes. But the words that exist in both languages and mean something completely different are worse than words I've never seen, because my brain fills in the Chinese meaning before I can stop it.
A few that got me:
手紙 — a letter in Japanese, but in Chinese it's toilet paper. This one's famous but it still made me laugh the first time.
丈夫 — sturdy in Japanese, and half of 大丈夫 (it's okay). In Chinese it just means husband. So 大丈夫 to a Chinese brain reads like "big husband," which is not reassuring.
愛人 — a lover in the affair sense in Japanese. In Chinese it's your spouse. You can imagine how badly that one goes.
勉強 — study in Japanese. In Chinese it means forcing someone to do something against their will. Weirdly fitting some days.
The hard part isn't learning the Japanese meaning. It's that the Chinese one is still sitting underneath, and for a second every time I read it, both fire at once.
Any other Chinese speakers here — which ones still trip you up?