This is a classic mistake many travelers make: they can travel in the winter or go somewhere without a pool and assume they don’t need swimwear. Just to understand that the weather is actually more beautiful than they expected, or if their friends know the great beach or lake nearby, always bring a pair of swimwear to make fun of.
Slippers are the most practical shoes to take with you on a trip: you need to take a shower in the hotel (because who knows how clean these baths really are?) and they do not take up much space in your bag.
Like swimwear, coats are not only useful in one season. If you fold them, they can be used as a temporary pillow or blanket on the plane, or if your hotel does not provide enough. But obviously you can also just use it as a coat if the weather for your summer vacation is not as warm as you expected.
First aid kit
When it comes to a first aid kit, many people tend to bring only a few strippers because they think it’s a waste of space in their suitcase and they probably don’t need anything anyway. But think about it: you don’t want to get stuck in a foreign country with a pulsating migraine and you have to find out where the nearest pharmacy is, or even go to a pharmacy just to look for something that could be equivalent to the medicine you usually take at home.