diff -up vim80/src/ops.c.cve0261 vim80/src/ops.c
--- vim80/src/ops.c.cve0261	2022-01-26 14:30:27.475308323 +0100
+++ vim80/src/ops.c	2022-01-26 14:34:16.650933713 +0100
@@ -636,23 +636,30 @@ block_insert(
 	    if (b_insert)
 	    {
 		off = (*mb_head_off)(oldp, oldp + offset + spaces);
+		spaces -= off;
+		count -= off;
 	    }
 	    else
 	    {
-		off = (*mb_off_next)(oldp, oldp + offset);
-		offset += off;
+		// spaces fill the gap, the character that's at the edge moves
+		// right
+		off = (*mb_head_off)(oldp, oldp + offset);
+		offset -= off;
 	    }
 	    spaces -= off;
 	    count -= off;
 	}
 #endif
 
-	newp = alloc_check((unsigned)(STRLEN(oldp)) + s_len + count + 1);
+	// Make sure the allocated size matches what is actually copied below.
+	newp = alloc(STRLEN(oldp) + spaces + s_len
+		    + (spaces > 0 && !bdp->is_short ? p_ts - spaces : 0)
+								  + count + 1);
 	if (newp == NULL)
 	    continue;
 
 	/* copy up to shifted part */
-	mch_memmove(newp, oldp, (size_t)(offset));
+	mch_memmove(newp, oldp, (size_t)offset);
 	oldp += offset;
 
 	/* insert pre-padding */
@@ -662,14 +669,21 @@ block_insert(
 	mch_memmove(newp + offset + spaces, s, (size_t)s_len);
 	offset += s_len;
 
-	if (spaces && !bdp->is_short)
+	if (spaces > 0 && !bdp->is_short)
 	{
-	    /* insert post-padding */
-	    vim_memset(newp + offset + spaces, ' ', (size_t)(p_ts - spaces));
-	    /* We're splitting a TAB, don't copy it. */
-	    oldp++;
-	    /* We allowed for that TAB, remember this now */
-	    count++;
+	    if (*oldp == TAB)
+	    {
+		// insert post-padding
+		vim_memset(newp + offset + spaces, ' ',
+						    (size_t)(p_ts - spaces));
+		// we're splitting a TAB, don't copy it
+		oldp++;
+		// We allowed for that TAB, remember this now
+		count++;
+	    }
+	    else
+		// Not a TAB, no extra spaces
+		count = spaces;
 	}
 
 	if (spaces > 0)
@@ -2738,9 +2752,9 @@ op_insert(oparg_T *oap, long count1)
 		oap->start_vcol = t;
 	    }
 	    else if (oap->op_type == OP_APPEND
-		      && oap->end.col
+		      && oap->start.col
 #ifdef FEAT_VIRTUALEDIT
-			    + oap->end.coladd
+			    + oap->start.coladd
 #endif
 			>= curbuf->b_op_start_orig.col
 #ifdef FEAT_VIRTUALEDIT
diff -up vim80/src/testdir/test_visual.vim.cve0261 vim80/src/testdir/test_visual.vim
--- vim80/src/testdir/test_visual.vim.cve0261	2022-01-26 14:30:27.476308325 +0100
+++ vim80/src/testdir/test_visual.vim	2022-01-26 14:36:03.482225225 +0100
@@ -254,3 +254,12 @@ func Test_virtual_replace2()
   %d_
   set bs&vim
 endfunc
+
+func Test_visual_block_append_invalid_char()
+  " this was going over the end of the line
+  new
+  call setline(1, ['	   let xxx', 'xxxxxˆ', 'xxxxxxxxxxx'])
+  exe "normal 0\<C-V>jjA-\<Esc>"
+  call assert_equal(['	-   let xxx', 'xxxxx   -ˆ', 'xxxxxxxx-xxx'], getline(1, 3))
+  bwipe!
+endfunc