I created a commit with mercurial and wrote a 260 character commit message (on command line, so I was driving blind and didn't use newlines).
I couldn't find any additional related logs on server in nginx/php-fpm or mysql logfiles.
```
host128:java cspeck$ hg ci -m "[insert 260-character message here]"
host128:java cspeck$ arc diff --update D12 --trace
libphutil loaded from '/Users/cspeck/Projects/phacility/libphutil/src'.
arcanist loaded from '/Users/cspeck/Projects/phacility/arcanist/src'.
Config: Reading user configuration file "/Users/cspeck/.arcrc"...
Config: Did not find system configuration at "/etc/arcconfig".
Working Copy: Reading .arcconfig from "/Users/cspeck/Source/enad/java/.arcconfig".
Working Copy: Path "/Users/cspeck/Source/enad/java" is part of `hg` working copy "/Users/cspeck/Source/enad/java".
Working Copy: Project root is at "/Users/cspeck/Source/enad/java".
Config: Did not find local configuration at "/Users/cspeck/Source/enad/java/.hg/arc/config".
>>> [0] <conduit> user.whoami() <bytes = 117>
>>> [1] <http> https://phab.mycompany.com/api/user.whoami
<<< [1] <http> 511,973 us
<<< [0] <conduit> 512,176 us
>>> [2] <exec> $ HGPLAIN=1 hg status
<<< [2] <exec> 387,786 us
>>> [3] <event> diff.didCollectChanges <listeners = 0>
<<< [3] <event> 54 us
>>> [4] <conduit> differential.query() <bytes = 146>
>>> [5] <http> https://phab.mycompany.com/api/differential.query
<<< [5] <http> 241,158 us
<<< [4] <conduit> 241,308 us
>>> [6] <conduit> differential.getcommitmessage() <bytes = 169>
>>> [7] <http> https://phab.mycompany.com/api/differential.getcommitmessage
<<< [7] <http> 86,217 us
<<< [6] <conduit> 86,375 us
>>> [8] <conduit> differential.parsecommitmessage() <bytes = 1147>
>>> [9] <http> https://phab.mycompany.com/api/differential.parsecommitmessage
<<< [9] <http> 65,885 us
<<< [8] <conduit> 66,027 us
>>> [10] <event> diff.didBuildMessage <listeners = 0>
<<< [10] <event> 32 us
>>> [11] <exec> $ nano '/var/folders/kw/22j3hs0j08v7lr6j448pm8lw0000gn/T/edit.93gu9d5flxs8kcsw/differential-update-comments'
<<< [11] <exec> 6,198,293 us
Linting...
>>> [12] <exec> $ HGPLAIN=1 hg help phase
<<< [12] <exec> 76,761 us
>>> [13] <exec> $ HGPLAIN=1 hg branch
<<< [13] <exec> 69,983 us
>>> [14] <exec> $ HGPLAIN=1 hg log --branch 'default' -r 'draft()' --style default
<<< [14] <exec> 267,610 us
>>> [15] <exec> $ HGPLAIN=1 hg parents --style default --rev '.'
<<< [15] <exec> 87,684 us
>>> [16] <exec> $ HGPLAIN=1 hg parents --style default --rev '7c78b5dfb01f'
<<< [16] <exec> 98,653 us
>>> [17] <exec> $ HGPLAIN=1 hg parents --style default --rev '0d164fee36f0'
<<< [17] <exec> 94,965 us
>>> [18] <exec> $ HGPLAIN=1 hg parents --style default --rev 'b7a2eb14716d'
<<< [18] <exec> 95,936 us
>>> [19] <exec> $ HGPLAIN=1 hg parents --style default --rev '6c34544708d2'
<<< [19] <exec> 95,276 us
>>> [20] <exec> $ HGPLAIN=1 hg parents --style default --rev '72e6647e79eb'
<<< [20] <exec> 96,503 us
>>> [21] <exec> $ HGPLAIN=1 hg parents --style default --rev 'ad0c9585c425'
<<< [21] <exec> 94,930 us
>>> [22] <exec> $ HGPLAIN=1 hg parents --style default --rev 'a7731f7a89c6'
<<< [22] <exec> 98,602 us
>>> [23] <exec> $ HGPLAIN=1 hg parents --style default --rev '98ea4a45a933'
<<< [23] <exec> 95,340 us
No lint engine configured for this project.
Running unit tests...
No unit test engine is configured for this project.
>>> [24] <exec> $ HGPLAIN=1 hg diff --git -U32767 --rev 'e9db07b69464' -- ''
<<< [24] <exec> 343,248 us
>>> [25] <exec> $ HGPLAIN=1 hg cat --rev 'e9db07b69464' --output '/var/folders/kw/22j3hs0j08v7lr6j448pm8lw0000gn/T/8cw2tdh1ohgc840w/%p' --
<<< [25] <exec> 79,573 us
>>> [26] <exec> $ HGPLAIN=1 hg cat --rev '.' --output '/var/folders/kw/22j3hs0j08v7lr6j448pm8lw0000gn/T/7t28q6jqe4socwwg/%p' --
<<< [26] <exec> 78,830 us
>>> [27] <exec> $ HGPLAIN=1 hg log -l 1 --template '{node}' -r 'e9db07b69464' --
<<< [27] <exec> 94,095 us
>>> [28] <exec> $ HGPLAIN=1 hg bookmarks
<<< [28] <exec> 77,347 us
>>> [29] <exec> $ HGPLAIN=1 hg svn info
<<< [29] <exec> 105,832 us
>>> [30] <conduit> repository.query() <bytes = 156>
>>> [31] <http> https://phab.mycompany.com/api/repository.query
<<< [31] <http> 141,133 us
<<< [30] <conduit> 141,288 us
>>> [32] <conduit> differential.creatediff() <bytes = 887782>
>>> [33] <http> https://phab.mycompany.com/api/differential.creatediff
<<< [33] <http> 804,968 us
<<< [32] <conduit> 805,127 us
>>> [34] <event> diff.wasCreated <listeners = 0>
<<< [34] <event> 36 us
>>> [35] <exec> $ HGPLAIN=1 hg log --template '{node}{rev}{author}{date|rfc822date}{branch}{tag}{parents}{desc}' --rev '('\''e9db07b69464'\''::. - '\''e9db07b69464'\'')' --branch 'default' --
<<< [35] <exec> 100,694 us
>>> [36] <exec> $ HGPLAIN=1 hg parents --template '{node}\n' --rev '98ea4a45a933a71760b3e1bb49adda25d8d7e89e'
>>> [37] <exec> $ HGPLAIN=1 hg parents --template '{node}\n' --rev '0d164fee36f0be99cfadfaebfb7ff0af46fdb7c7'
<<< [36] <exec> 86,285 us
<<< [37] <exec> 85,842 us
>>> [38] <conduit> differential.setdiffproperty() <bytes = 9567>
>>> [39] <http> https://phab.mycompany.com/api/differential.setdiffproperty
<<< [39] <http> 156,501 us
<<< [38] <conduit> 156,656 us
>>> [40] <conduit> differential.updaterevision() <bytes = 1568>
>>> [41] <http> https://phab.mycompany.com/api/differential.updaterevision
<<< [41] <http> 193,675 us
<<< [40] <conduit> 193,814 us
[2015-04-23 21:58:49] EXCEPTION: (HTTPFutureHTTPResponseStatus) [HTTP/500] Internal Server Error
{"result":null,"error_code":"ERR-CONDUIT-CORE","error_info":"#1406: Data too long for column 'description' at row 1"} at [<phutil>/src/future/http/BaseHTTPFuture.php:337]
arcanist(head=master, ref.master=9ddf37b9ee3e), phutil(head=master, ref.master=20d43107280d)
#0 BaseHTTPFuture::parseRawHTTPResponse(string) called at [<phutil>/src/future/http/HTTPSFuture.php:414]
#1 HTTPSFuture::isReady() called at [<phutil>/src/future/Future.php:39]
#2 Future::resolve(NULL) called at [<phutil>/src/future/FutureProxy.php:36]
#3 FutureProxy::resolve() called at [<phutil>/src/conduit/ConduitClient.php:57]
#4 ConduitClient::callMethodSynchronous(string, array) called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:525]
#5 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:378]
```