Trying to mangle some characters resulted in a weird error message:

$ echo hello | tr [:lower:] [:upper:]
Bad string

Huh? Before debugging any further, searching the interwebs returns quite a few results, of course, so let's look at our options then:

$ type tr
tr is /usr/bin/tr

$ find /usr -type f -perm -0500 -name tr -ls 2>/dev/null
32054   11 -rwxr-xr-x   1 root bin  9916 Jan 23  2005 /usr/ucb/tr
16674   19 -r-xr-xr-x   1 root bin 18540 Jan 23  2005 /usr/xpg6/bin/tr
  410   20 -r-xr-xr-x   1 root bin 19400 Jan 23  2005 /usr/bin/tr
75251   19 -r-xr-xr-x   1 root bin 18520 Jan 23  2005 /usr/xpg4/bin/tr

Besides our default from SUNWcsu, we have three other versions of tr(1) available. The UCB version tries do do...something:

$ echo hello | /usr/ucb/tr [:lower:] [:upper:]
heuup

Apparently it replaces each character (position) literally, but fails to recognize the bracket expressions. Since the UCB tools were removed in later versions anyway, let's skip that for now. The two X/Open versions seem to manage:

$ echo hello | /usr/xpg6/bin/tr [:lower:] [:upper:]
HELLO

$ echo hello | /usr/xpg4/bin/tr [:lower:] [:upper:]
HELLO

But why wouldn't it work with the SUNWcsu version? truss(1) reports a missing file, but this turns out to be a red herring:

$ echo hello | truss -elfda tr [[:lower:]] [[:upper:]]
Base time stamp:  1481011767.7308  [ Tue Dec  6 09:09:27 MET 2016 ]
26125/1:         0.0000 execve("/usr/bin/tr", 0xFFBFFC9C, 0xFFBFFCAC)  argc = 3
26125/1:         argv: tr [[:lower:]] [[:upper:]]
26125/1:         envp: LC_MONETARY=en_GB.ISO8859-15 TERM=xterm SHELL=/bin/bash
26125/1:          LC_NUMERIC=en_GB.ISO8859-15 LC_ALL=en_US.UTF-8
26125/1:          LC_MESSAGES=C LC_COLLATE=en_GB.ISO8859-15 LANG=en_US.UTF-8
26125/1:          LC_CTYPE=en_GB.ISO8859-1 LC_TIME=en_GB.ISO8859-15
[...]
26125/1:         0.0061 stat64("/usr/lib/locale/en_US.UTF-8/libc.so.1", 0xFFBFE8D0) Err#2 ENOENT
26125/1:         0.0063 open("/usr/lib/locale/en_US.UTF-8/LC_MESSAGES/SUNW_OST_OSCMD.mo", O_RDONLY) Err#2 ENOENT
26125/1:         0.0064 fstat64(2, 0xFFBFEA38)                          = 0
Bad string
26125/1:         0.0064 write(2, " B a d   s t r i n g\n", 11)          = 11
26125/1:         0.0065 _exit(1)

(Un)fortunately I had my share of weird experiences with character encodings and the like. And indeed, if we use a single-byte locale, /usr/bin/tr works just fine:

$ echo $LC_ALL
en_US.UTF-8

$ echo hello | LC_ALL=en_US tr [[:lower:]] [[:upper:]]
HELLO

Another workaround would be to use another expression, if possible:

$ echo hello | tr [a-z] [A-Z]
HELLO

In newer SunOS versions, /usr/bin/tr has been fixed and works as expected.