I see a lot of clients writing "however" when they mean "but." The truth is that they do mean the same thing. However, they are not the same kind of word. "But" is a coordinating conjunction. It can link two independent clauses together. "However" is not. "However" is more flexible and has more possible meanings.

"I ate my apple. I was still hungry, however. I didn't eat again."

This is equivalent to, "I ate my apple but I was still hungry. I didn't eat again." The point of the sentences is that the apple didn't satisfy the speaker's hunger and not eating again is just an extra detail.

"I ate my apple. I was still hungry. However, I didn't eat again."

This is equivalent to, "I ate my apple. I was still hungry but I didn't eat again." The point of the sentences is that the speaker decided not to eat even though he or she was hungry and eating the apple is just an extra detail.

So if we have a run-on sentence that goes "I ate my apple. I was still hungry, however, I didn't eat again," then we can't tell exactly what the original writer was trying to say.

Think of "but" as an arrow that only points to the right →. Think of "however" as an arrow that can point to the right → or to the left ← depending on context. Because "however" can mean different things depending on how it is placed in the sentence, we must be more careful about where we place it than with "but."