What was that all about in the St Mirren game?

Was it my imagination or did Celtic bring on 6 subs in the St Mirren game?
If so, how was that allowed as only 5 subs are permitted per game?
Or is there a rule in place to allow 6 subs under certain circumstances?

The Current Bun has the following report about it:

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Ok, well there we go, that’s cleared that up, cheers.