ことある - I have
食べたことある - I've eaten it before
So what does 食べたことあった mean?
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I would describe 「V した＋ことがある」as to convey having a certain experience determined by the verb. In English it is usually rendered as "have done something". In much the same vein, 「V した＋ことがあった」also talks about having that experience, but the having itself is talked about as in the past. With あった there is a distance between that experience and the current discussion. Let's look at some examples:
I had eaten (at a similar restaurant) in Fukuoka. That's why I visited/tried this place.
Here the English translation conveniently matches the Japanese original in tense. Just like the Japanese sentence, you backshift the English version, "have eaten" -> "had eaten". Note that not all 「V したことがあった」sentences can be rendered this way. The speaker talks about having had an experience (eating at a restaurant similar to the one they are reviewing), and that experience having happened prior to their visit to this new restaurant is the reason they talk about that experience in the comparatively more distant past.
Here the experience — one of their friends stealing fruits to share with the group — is being reminisced about fondly/nostalgically, thus the distance between the conversation at hand and the experience talked about.
People also say 「何か変わったことあった？」to get updates.