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.
It's an Mac app, so jailbroken doesn't apply.