Re: most serious I’ve seen – There was a while during the 1970’s when our parish was using parishioner-baked bread for Communion hosts, and they were including honey and other extraneous substances. I’m not totally sure that they avoided leavening, either, with all that sugar in their bread. It happened during the summer and mysteriously stopped after a few Sundays, so I think our pastor must have been away but stopped it when he came back. He was very good at putting the kibosh on anything against the rules.
Re: hymns – If music is used during Mass (ie, not before or after Mass), it must be sacred music, not secular music. During the Sixties and Seventies, a lot of people thought that intending a secular song to be dedicated to God would magically make it sacred. That’s not true. (It’s also insulting to the purposes of secular music, to pretend that waving a magic wand of performer intention should supersede the intentions of the composer and songwriter.)
This still does not constitute liturgical abuse – unless you do something even more serious, like using a secular song as a replacement for the responsorial psalm, or for one of the parts of the Mass. But the only way you could actually invalidate Mass would be to use a secular song to replace the Eucharistic Canon. So we can count our blessings there.
However, a lot of people don’t know that the Church strictly forbids the use of recorded music during Mass (MIDI is okay; but what is played or sung during Mass has to be a live performance, because music is an offering and prayer to God, not wallpaper).
Also, nobody should be having secular performances of any kind in a church (outside Mass) without the Blessed Sacrament being removed from the tabernacle. (The music or performance doesn’t have to be offensive; obviously you shouldn’t have an offensive or evil performance in church at all! It’s just that using the church as a gathering hall for non-religious purposes is different from going to church as a house of prayer.) It is a pretty serious abuse of the Blessed Sacrament, but it’s not a liturgical abuse.
The best way I’ve seen churches deal with it is that, if people want an explicitly secular song at their wedding or funeral, the parish has the song performed on the steps outside. For example, a lot of folks get their secular Scottish bagpipe and drum songs that way, while anything during the actual wedding or funeral Mass is strictly sacred in nature. There is a lot of blank space during the time before a coffin enters the church or a bride gets herself off the steps where people are taking photos to actually go to the reception, and music helps fill the time.
And that’s probably enough about that… But yeah, parish musicians have to know about this stuff and think about it, especially since it goes against the way things are done in non-Catholic settings. Sometimes it comes as a rude surprise if people don’t know about it.