I think it is a big problem to have so many teachers that aren’t qualified to teach any subject to someone. Both teacher and students are affected, I may say; the teacher sometimes is called out to give classes that isn’t his or her subject suddenly and have to go anyway, with even knowing the content he/she is going to teach in that subject that day.
It causes revolt in the parts, the teachers don’t know what to do, they don’t have the right knowledge for that, because they didn’t study for that; and the students, because it is bad for their learning process, and they can feel the insecurity in the unprepared teacher.

“2009 School Census data tabulated by the National Institute of Educational Studies Teixeira (Inep) reveal that just over half (53.3%) of teachers working at the school in public education are compatible with the discipline they teach. The total is 366,757. The final grades of primary school stage in which all the materials begin to be given by teachers of specific areas, the ratio is even lower: 46.7% of 617 571 teachers.
And the worst thing is the Studies show that the states with the largest number of teachers without adequate training (including here lay teachers who work in networks) are those with the worst performance of the Index of Basic Education Development.”
But there are controversial opinions that say it is better for the students to have the content with a teacher that doesn’t teach the target subject than nothing and lose the theme for that day. I see no gain in this way, the students won’t really understand if the teacher isn’t able to explain and the content will fail and it will be lost anyway.
The solution probably will be more investments in education to help in the lack of teachers; there is a lot of demotivation behind all of it which leads the people to other careers.