2. Underestimation of the preparation time
First of all and as mentioned before, do not underestimate the time it will take you to make your decision. Take your time for this process and realize why you need this time out, what you want to fill it with and if you may even want to achieve something specific during your trip. Give your dream enough time to prosper, so you can enjoy it even more. If you have come to a conclusion and made up your mind, the next step is to develop a financial concept for your absence in the company and also concerning your insurances. Some models that work with saving up time or money to make your sabbatical possible might take up to six years of preparation.
Besides the time for your own planning you should also give your employer enough time to react to your proposal. A well prepared conversation is of course essential to convince your employer and it also makes it easier for him/ her to realize your proposal as you have thought through possibilities on how to finance it and how to divide your work load. Nevertheless your boss still needs time to make a decision, to fill out a request for your proposal and to coordinate the distribution of your responsibilities and tasks between your colleagues.
In addition to that you should also plan ahead to have enough time for possibly necessary vaccinations. Especially if you want to travel the world visiting different countries, it is very important to deal with this topic early enough, as sometimes several injections are needed.