Here is an interesting problem:

While using an application you edit a record with the intention to change the value in a drop-down list. Available options are A, B, C and B is currently selected. So you change that to C and save. Now, you change your mind and try to make it B again, but option B is now missing from the available options.

Here is why this happens: the admin has edited the available options for this drop-down list and removed option B. Records using this value still show it, but after selecting a different option there is no need to show it again.

Of course this is way too unfriendly and weird. I could only think of three solutions to this problem:

1. Do not allow the admin to remove options used by at least one record. The problem here is that if the admin makes a mistake while creating options, there is no way to fix it other than manually editing all records using the particular option.
2. Make the admin select an option that will take the removed option’s place. E.g. “Remove option B and put option C in all records using it”. This is not always easy: imagine a field “How did you hear about us” with starting options “Internet”, “Magazine”. Now the admin wants to remove “Internet” and add “Internet: ad”, “Internet: web site”, “Internet: search”. He can’t really replace the old option with one of the new three.
3. Add a small descriptive text to removed values. So the user would see these options: “A”, “B [removed]“, “C”. This way, no one has decided the new value for him and he is informed that option B is now deprecated. He can either select a new value or leave it as is. Moreover the admin has no problems in removing options.

I went with the 3rd solution which, while not super user-friendly, is at least flexible.