An error

[fionacat]

Error : Collection was modified; enumeration operation may not execute.

... okay then.

Comments

[terrycloth]

It's hard (for a computer) to do something to each member of a set if the set keeps changing while it's trying to enumerate them.

There are ways to deal with it (especially if you know how the set is going to be changing while you're enumerating it) but some standard enumerations just fail.