Not repeatable in mysql-5.1 . Using LAST_DAY() in ORDER BY with non-date argument asserts as follows:

mysqld: filesort.cc:854: void make_sortkey(SORTPARAM*, uchar*, uchar*): Assertion `maybe_null && item->null_value' failed.

#8 0x006b0d98 in __assert_fail () from /lib/libc.so.6
#9 0x08386dc0 in make_sortkey (param=0xb6bccbb8,
to=0xb6a041bc "\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245\245h4z\025e", ref_pos=0xb6bcc998 "") at filesort.cc:854
#10 0x08386429 in find_all_keys (param=0xb6bccbb8, select=0xb6a06398, sort_keys=0xb6a04188, buffpek_pointers=0xb6bcca30, tempfile=0xb6bccaf4, indexfile=0x0)
at filesort.cc:620
#11 0x08385120 in filesort (thd=0x9c59090, table=0xb6a024f0, sortorder=0xb6a064a0, s_length=1, select=0xb6a06398, max_rows=18446744073709551615,
sort_positions=false, examined_rows=0xb6bccd20) at filesort.cc:245
#12 0x082f5476 in create_sort_index (thd=0x9c59090, join=0xb6a04b58, order=0xb6a04aa8, filesort_limit=18446744073709551615,
select_limit=18446744073709551615, is_order_by=false) at sql_select.cc:13890
#13 0x082d918e in JOIN::exec (this=0xb6a04b58) at sql_select.cc:2277
#14 0x082d9c62 in mysql_select (thd=0x9c59090, rref_pointer_array=0x9c5a570, tables=0xb6a04788, wild_num=1, fields=..., conds=0x0, og_num=1,
order=0xb6a04aa8, group=0x0, having=0x0, proc_param=0x0, select_options=2147764736, result=0xb6a04b40, unit=0x9c5a208, select_lex=0x9c5a478)
at sql_select.cc:2544
#15 0x082d264e in handle_select (thd=0x9c59090, lex=0x9c5a1ac, result=0xb6a04b40, setup_tables_done_option=0) at sql_select.cc:269
#16 0x0827a78f in execute_sqlcom_select (thd=0x9c59090, all_tables=0xb6a04788) at sql_parse.cc:5129
#17 0x08271646 in mysql_execute_command (thd=0x9c59090) at sql_parse.cc:2278
#18 0x0827c948 in mysql_parse (thd=0x9c59090, rawbuf=0xb6a04628 "SELECT * FROM t1 ORDER BY LAST_DAY ( f2 )", length=41, found_semicolon=0xb6bce210)
at sql_parse.cc:6054
#19 0x0826f2a0 in dispatch_command (command=COM_QUERY, thd=0x9c59090, packet=0x9c9e539 "SELECT * FROM t1 ORDER BY LAST_DAY ( f2 ) ", packet_length=43)
at sql_parse.cc:1249
#20 0x0826e4d0 in do_command (thd=0x9c59090) at sql_parse.cc:889
#21 0x0826c815 in handle_one_connection (arg=0x9c59090) at sql_connect.cc:1136
#22 0x00821919 in start_thread () from /lib/libpthread.so.0
#23 0x0076acce in clone () from /lib/libc.so.6

test case:

CREATE TABLE t1 ( f2 int NOT NULL) ;
INSERT INTO t1 VALUES (0),(0);

SELECT * FROM t1 ORDER BY LAST_DAY ( f2 ) ;