A user is reporting a crash, and I cannot see how it could possibly be happening.
The code in question (in frame #18 below) is this:
NSFont *font = [NSFont fontWithName:@"Helvetica Neue" size:13];
// some other code
NSGlyph typicalGlyph = [font glyphWithName:@"e"];
The font object is not nil. If it were, it would have crashed before this point.
Here's the relevant part of the crash log:
Code Type: X86-64 (Native)
Parent Process: launchd [193]
User ID: 502
Date/Time: 2013-12-04 20:17:29.086 +0100
OS Version: Mac OS X 10.8.5 (12F45)
Report Version: 10
Interval Since Last Report: 326397 sec
Crashes Since Last Report: 236
Per-App Interval Since Last Report: 67 sec
Per-App Crashes Since Last Report: 10
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_CRASH (SIGSEGV)
Exception Codes: 0x0000000000000000, 0x0000000000000000
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 libsystem_kernel.dylib 0x00007fff897b8686 mach_msg_trap + 10
1 libsystem_kernel.dylib 0x00007fff897b7c42 mach_msg + 70
2 com.apple.CoreGraphics 0x00007fff83516752 _CGSCacheGetUniqueID + 177
3 com.apple.CoreGraphics 0x00007fff83516647 CGSCacheGetUniqueID + 158
4 com.apple.CoreGraphics 0x00007fff83515d8a CGFontGetGFID + 328
5 com.apple.CoreGraphics 0x00007fff83515c1a CGFontGetIdentifier + 31
6 com.apple.CoreGraphics 0x00007fff835136a7 CGFontCacheRemoveFont + 114
7 com.apple.CoreGraphics 0x00007fff835134fc font_finalize + 32
8 com.apple.CoreFoundation 0x00007fff890573df CFRelease + 511
9 com.apple.CoreFoundation 0x00007fff8907e655 -[__NSArrayM dealloc] + 229
10 com.apple.CoreFoundation 0x00007fff8905728a CFRelease + 170
11 com.apple.CoreGraphics 0x00007fff835125ce CGFontCreateFontsWithURL + 615
12 com.apple.CoreText 0x00007fff8f3eed71 CreateFontWithFontURL(__CFURL const*) + 37
13 com.apple.CoreText 0x00007fff8f3eec1a TCGFontCache::CopyFont(__CFURL const*) const + 110
14 com.apple.CoreText 0x00007fff8f3eea92 TBaseFont::CopyNativeFont() const + 48
15 com.apple.CoreText 0x00007fff8f43a806 TFont::GetGlyphWithName(__CFString const*) const + 54
16 com.apple.CoreText 0x00007fff8f435351 CTFontGetGlyphWithName + 41
17 com.apple.AppKit 0x00007fff87d353bd -[NSFont glyphWithName:] + 63
18 com.dayoneapp.dayone 0x0000000104e7d865 0x104e4f000 + 190565
19 com.dayoneapp.dayone 0x0000000104e7dbcb 0x104e4f000 + 191435
20 com.dayoneapp.dayone 0x0000000104e7e212 0x104e4f000 + 193042
21 com.dayoneapp.dayone 0x0000000104e7e780 0x104e4f000 + 194432
22 com.dayoneapp.dayone 0x0000000104e6fbfa 0x104e4f000 + 134138
23 com.apple.AppKit 0x00007fff87b39ae2 -[NSTableView _sendDelegateHeightOfRow:] + 130
24 com.apple.AppKit 0x00007fff87b3989b -[NSTableView _uncachedRectHeightOfRow:] + 197
25 com.apple.AppKit 0x00007fff87b3973a -[_NSTableRowHeightStorage _cacheRowHeights] + 241
26 com.apple.AppKit 0x00007fff87af3e45 -[_NSTableRowHeightStorage _ensureRowHeights] + 80
27 com.apple.AppKit 0x00007fff87af3d1b -[_NSTableRowHeightStorage computeTableHeightForNumberOfRows:] + 62
28 com.apple.AppKit 0x00007fff87af3768 -[NSTableView _minimumFrameSize] + 74
29 com.apple.AppKit 0x00007fff87b10409 -[NSTableView tile] + 240
30 com.apple.AppKit 0x00007fff87b18570 -[NSTableView layout] + 110
31 com.apple.AppKit 0x00007fff87acbe65 -[NSView _layoutSubtreeHeedingRecursionGuard:] + 112
32 com.apple.CoreFoundation 0x00007fff890da4a6 __NSArrayEnumerate + 582
33 com.apple.AppKit 0x00007fff87acbfc6 -[NSView _layoutSubtreeHeedingRecursionGuard:] + 465
34 com.apple.CoreFoundation 0x00007fff890da4a6 __NSArrayEnumerate + 582
35 com.apple.AppKit 0x00007fff87acbfc6 -[NSView _layoutSubtreeHeedingRecursionGuard:] + 465
36 com.apple.CoreFoundation 0x00007fff890da4a6 __NSArrayEnumerate + 582
37 com.apple.AppKit 0x00007fff87acbfc6 -[NSView _layoutSubtreeHeedingRecursionGuard:] + 465
38 com.apple.CoreFoundation 0x00007fff890da4a6 __NSArrayEnumerate + 582
39 com.apple.AppKit 0x00007fff87acbfc6 -[NSView _layoutSubtreeHeedingRecursionGuard:] + 465
40 com.apple.CoreFoundation 0x00007fff890da4a6 __NSArrayEnumerate + 582
41 com.apple.AppKit 0x00007fff87acbfc6 -[NSView _layoutSubtreeHeedingRecursionGuard:] + 465
42 com.apple.CoreFoundation 0x00007fff890da4a6 __NSArrayEnumerate + 582
43 com.apple.AppKit 0x00007fff87acbfc6 -[NSView _layoutSubtreeHeedingRecursionGuard:] + 465
44 com.apple.CoreFoundation 0x00007fff890da4a6 __NSArrayEnumerate + 582
45 com.apple.AppKit 0x00007fff87acbfc6 -[NSView _layoutSubtreeHeedingRecursionGuard:] + 465
46 com.apple.AppKit 0x00007fff87acbcfe -[NSView layoutSubtreeIfNeeded] + 615
47 com.apple.AppKit 0x00007fff87ac74ac -[NSWindow(NSConstraintBasedLayout) layoutIfNeeded] + 201
48 com.apple.AppKit 0x00007fff879c10a8 _handleWindowNeedsDisplayOrLayoutOrUpdateConstraints + 446
49 com.apple.AppKit 0x00007fff87f8c901 __83-[NSWindow _postWindowNeedsDisplayOrLayoutOrUpdateConstraintsUnlessPostingDisabled]_block_invoke_01208 + 46
50 com.apple.CoreFoundation 0x00007fff890a8417 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
51 com.apple.CoreFoundation 0x00007fff890a8381 __CFRunLoopDoObservers + 369
52 com.apple.CoreFoundation 0x00007fff890837b8 __CFRunLoopRun + 728
53 com.apple.CoreFoundation 0x00007fff890830e2 CFRunLoopRunSpecific + 290
54 com.apple.HIToolbox 0x00007fff88d7ceb4 RunCurrentEventLoopInMode + 209
55 com.apple.HIToolbox 0x00007fff88d7cc52 ReceiveNextEventCommon + 356
56 com.apple.HIToolbox 0x00007fff88d7cae3 BlockUntilNextEventMatchingListInMode + 62
57 com.apple.AppKit 0x00007fff879be533 _DPSNextEvent + 685
58 com.apple.AppKit 0x00007fff879bddf2 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 128
59 com.apple.AppKit 0x00007fff879b51a3 -[NSApplication run] + 517
60 com.apple.AppKit 0x00007fff87959bd6 NSApplicationMain + 869
61 com.dayoneapp.dayone 0x0000000104e516e4 0x104e4f000 + 9956
// More threads, but nothing touching fonts or drawing code.
There are Cydia hacks that mess up with Fonts, I've seen this type of crash in the past only for jailbroken devices. Do you have a way to know if this user has a jailbroken device? Hockeyapp tells you.