there are times when; sometimesAfter a verb in dictionary form, ことがある means that an action or event happens occasionally: 変わることがある, “may sometimes change.” After a verb in past form, it means “have done ... before,” but this often appears as ことがある or, in polite speech, ことがあります. It can also follow a noun-modifying clause to mean “there is something that ...,” as in 話したいことがある, “there is something I want to talk about.”
JA: その取り決めは、予告なしに変わることがある。
EN: That arrangement may sometimes change without notice.