diff --git a/desktop/api.cpp b/desktop/api.cpp
index acf7ca8..030a6be 100644
--- a/desktop/api.cpp
+++ b/desktop/api.cpp
@@ -268,7 +268,8 @@ uintptr_t APISyscallCheckForCrash(uintptr_t argument0, uintptr_t argument1, uint
 	uintptr_t returnValue = _APISyscall(argument0, argument1, argument2, unused, argument3, argument4);
 	EsProcessState state;
 	_APISyscall(ES_SYSCALL_PROCESS_GET_STATE, ES_CURRENT_PROCESS, (uintptr_t) &state, 0, 0, 0);
-	while (state.flags & ES_PROCESS_STATE_PAUSED_FROM_CRASH);
+	volatile int x = 1;
+	if (state.flags & ES_PROCESS_STATE_PAUSED_FROM_CRASH) while (x);
 	return returnValue;
 }
 #endif
@@ -1604,6 +1605,10 @@ void EsPanic(const char *cFormat, ...) {
 	size_t bytes = EsStringFormatV(buffer, sizeof(buffer), cFormat, arguments); 
 	va_end(arguments);
 	EsPrintDirect(buffer, bytes);
+#ifdef PAUSE_ON_USERLAND_CRASH
+	volatile int x = 1;
+	while (x);
+#endif
 	EsSyscall(ES_SYSCALL_PROCESS_CRASH, 0, 0, 0, 0);
 }
 
diff --git a/desktop/textbox.cpp b/desktop/textbox.cpp
index 5b28edd..50bdf35 100644
--- a/desktop/textbox.cpp
+++ b/desktop/textbox.cpp
@@ -263,6 +263,9 @@ void TextboxSetActiveLine(EsTextbox *textbox, int lineIndex) {
 		return;
 	}
 
+	EsAssert(lineIndex >= -1 && lineIndex < (int) textbox->lines.Length());
+	EsPrint("TextboxSetActiveLine %i\n", lineIndex);
+
 	if (lineIndex == -1) {
 		int32_t lineBytesDelta = textbox->activeLineBytes - textbox->activeLineOldBytes;
 
@@ -277,6 +280,7 @@ void TextboxSetActiveLine(EsTextbox *textbox, int lineIndex) {
 				lineBytesDelta,
 				false);
 		textbox->dataBytes += lineBytesDelta;
+		EsAssert(textbox->dataBytes >= 0);
 
 		// Step 3: Copy the active line back into the data buffer.
 
@@ -828,10 +832,24 @@ void TextboxUndoItemCallback(const void *item, EsUndoManager *manager, EsMessage
 void EsTextboxInsert(EsTextbox *textbox, const char *string, ptrdiff_t stringBytes, bool sendUpdatedMessage) {
 	EsMessageMutexCheck();
 
+	EsPrint("EsTextboxInsert \"%s\" at %d:%d->%d:%d.\n", 
+			stringBytes, string, textbox->carets[0].line, textbox->carets[0].byte, textbox->carets[1].line, textbox->carets[1].byte);
+
+#if 0
+	for (uintptr_t i = 0; i < textbox->lines.Length(); i++) {
+		EsPrint("line %d %d '%s'\n", i, textbox->lines[i].lengthBytes, textbox->lines[i].lengthBytes, GET_BUFFER(&textbox->lines[i]));
+	}
+#endif
+
 	if (string >= textbox->activeLine && string < textbox->activeLine + textbox->activeLineAllocated) {
 		EsAssert(false);
 	}
 
+	EsAssert(textbox->carets[0].line >= 0 && textbox->carets[0].line < (int32_t) textbox->lines.Length()
+			&& textbox->carets[0].byte >= 0 && textbox->carets[0].byte <= textbox->lines[textbox->carets[0].line].lengthBytes);
+	EsAssert(textbox->carets[1].line >= 0 && textbox->carets[1].line < (int32_t) textbox->lines.Length()
+			&& textbox->carets[1].byte >= 0 && textbox->carets[1].byte <= textbox->lines[textbox->carets[1].line].lengthBytes);
+
 	// EsPerformanceTimerPush();
 	// double measureLineTime = 0;
 
@@ -901,6 +919,7 @@ void EsTextboxInsert(EsTextbox *textbox, const char *string, ptrdiff_t stringByt
 			EsMemoryMove(textbox->activeLine + deleteTo.byte, textbox->activeLine + line->lengthBytes, deltaBytes, false);
 			textbox->activeLineBytes += deltaBytes;
 			line->lengthBytes += deltaBytes;
+			EsAssert(line->lengthBytes >= 0);
 
 			// Step 6: Update the longest line.
 
@@ -928,12 +947,14 @@ void EsTextboxInsert(EsTextbox *textbox, const char *string, ptrdiff_t stringByt
 
 			EsMemoryMove(textbox->data + deleteTo.byte + textbox->lines[deleteTo.line].offset, textbox->data + textbox->dataBytes, deltaBytes, false);
 			textbox->dataBytes += deltaBytes;
+			EsAssert(textbox->dataBytes >= 0);
 
 			// Step 6: Merged the joined lines.
 
 			DocumentLine *firstLine = &textbox->lines[deleteFrom.line];
 			firstLine->lengthBytes = textbox->lines[deleteTo.line].lengthBytes - deleteTo.byte + deleteFrom.byte;
 			firstLine->lengthWidth = TextGetStringWidth(textbox, &textbox->textStyle, textbox->data + firstLine->offset, firstLine->lengthBytes);
+			EsAssert(firstLine->lengthBytes >= 0);
 
 			// Step 7: Remove the deleted lines and update the textbox.
 
@@ -1011,6 +1032,7 @@ void EsTextboxInsert(EsTextbox *textbox, const char *string, ptrdiff_t stringByt
 			EsAssert(added == bytesToInsert); // Added incorrect number of bytes in EsTextboxInsert.
 
 			line->lengthBytes += bytesToInsert;
+			EsAssert(line->lengthBytes >= 0);
 
 			// Step 3: Update the carets, line width, and repaint it.
 
@@ -1032,11 +1054,13 @@ void EsTextboxInsert(EsTextbox *textbox, const char *string, ptrdiff_t stringByt
 			TextboxBufferResize((void **) &textbox->data, &textbox->dataAllocated, textbox->dataBytes + bytesToInsert, 1);
 			EsMemoryMove(textbox->data + byteOffset, textbox->data + textbox->dataBytes, bytesToInsert, false);
 			textbox->dataBytes += bytesToInsert;
+			EsAssert(textbox->dataBytes >= 0);
 
 			// Step 3: Truncate the insertion line.
 
 			int32_t truncation = line->lengthBytes - insertionPoint.byte;
 			line->lengthBytes = insertionPoint.byte;
+			EsAssert(line->lengthBytes >= 0);
 
 			// Step 4: Add the new lines.
 
@@ -1070,6 +1094,8 @@ void EsTextboxInsert(EsTextbox *textbox, const char *string, ptrdiff_t stringByt
 					line->lengthBytes += truncation;
 				}
 
+				EsAssert(line->lengthBytes >= 0);
+
 				// Step 4c: Update the line's width.
 
 				// EsPerformanceTimerPush(); 
diff --git a/shared/common.cpp b/shared/common.cpp
index 5602607..a8956e2 100644
--- a/shared/common.cpp
+++ b/shared/common.cpp
@@ -1287,7 +1287,7 @@ void EsMemoryMove(void *_start, void *_end, intptr_t amount, bool zeroEmptySpace
 	uint8_t *end = (uint8_t *) _end;
 
 	if (end < start) {
-		EsPrint("MemoryMove end < start: %x %x %x %d\n", start, end, amount, zeroEmptySpace);
+		EsPanic("MemoryMove end < start: %x %x %x %d\n", start, end, amount, zeroEmptySpace);
 		return;
 	}
 
diff --git a/util/build.c b/util/build.c
index 8617933..e5a2c44 100644
--- a/util/build.c
+++ b/util/build.c
@@ -1300,6 +1300,8 @@ void DoCommand(const char *l) {
 		BuildAndRun(OPTIMISE_FULL, true /* compile */, DEBUG_NONE /* debug */, EMULATOR_QEMU, LOG_NORMAL);
 	} else if (0 == strcmp(l, "kno")) {
 		BuildAndRun(OPTIMISE_ON, true /* compile */, DEBUG_NONE /* debug */, EMULATOR_QEMU, LOG_NORMAL);
+	} else if (0 == strcmp(l, "kd")) {
+		BuildAndRun(OPTIMISE_OFF, true /* compile */, DEBUG_NONE /* debug */, EMULATOR_QEMU, LOG_NORMAL);
 	} else if (0 == strcmp(l, "klv")) {
 		BuildAndRun(OPTIMISE_FULL, true /* compile */, DEBUG_NONE /* debug */, EMULATOR_QEMU, LOG_VERBOSE);
 	} else if (0 == strcmp(l, "tlv")) {