The Browser Console/Toolbox fails opening with various kind of exceptions.
Here is at least two I saw:

Error: Connection closed, pending request to server26.conn0.content-process71/console2, type getCachedMessages failed

Request stack:
request@resource://devtools/shared/protocol/Front.js:256:14
generateRequestMethods/</frontProto[name]@resource://devtools/shared/protocol/Front/FrontClassWithSpec.js:47:19
getCachedMessages@resource://devtools/client/fronts/webconsole.js:312:34
module.exports@resource://devtools/shared/resources/legacy-listeners/error-messages.js:43:44

and:

Error: Protocol error (TypeError): can't access property "dbg", this._thread is undefined from: server14.conn0.windowGlobal15032385537/consoleActor2 (resource://devtools/server/actors/utils/TabSources.js:167:28)

Error while calling actor 'console's method 'getCachedMessages' can't access property "dbg", this._thread is undefined Actor.js:91:13
getSourceActorByInternalSourceId@resource://devtools/server/actors/utils/TabSources.js:167:28
getActorIdForInternalSourceId@resource://devtools/server/actors/webconsole.js:1627:44
preparePageErrorForRemote@resource://devtools/server/actors/webconsole.js:1713:22
getCachedMessages@resource://devtools/server/actors/webconsole.js:974:31
handler@resource://devtools/shared/protocol/Actor.js:162:37
onPacket@resource://devtools/server/devtools-server-connection.js:379:58
_onPacketReceived@resource://devtools/shared/transport/js-window-actor-transport.js:46:18
emit@resource://gre/modules/EventEmitter.jsm:160:20
receiveMessage@resource://devtools/server/connectors/js-window-actor/DevToolsFrameChild.jsm:388:21

I reproduce them by closing and reopening the Browser Console on a Nightly build, with many tabs and opened for a least a day.

Once you get one of these two, you will always have it, everytime you re-try opening it.
These exceptions are only visible via about:devtools-toolbox?type=process, opened in a tab. For some very surprising reason, this toolbox works fine, whereas it is also a multiprocess toolbox..!

The severity field is not set for this bug.
:ochameau, could you have a look please?

For more information, please visit auto_nag documentation.

This is likely fixed by work done in Bug 1120863 - Browser toolbox starts blank and doesn't function with pending updates
Feel free to reopen if you are still experiencing this issue.

Honza